Because weather is the most powerful choice. Duh.
A quick look at the team:
Let's go in-depth
Tyranitar (M) - Leftovers
Trait: Sand Stream
Nature: Sassy
EVs: 252 HP/64 SAtk/192 SDef
A Sassy nature with 192 SDef EVs and the SDef boost from Sand Stream allows it to set up SR on almost any special lead. It packs quite a punch, with Crunch as its main STAB move to kill enemys like Reuniclus and Celebi. Fire Blast is for the frequent amount of Steel pokemon used as leads (Ferrothorn, Skarmory, Forretress), and can also kill Scizor if it doesn't has Superpower or Brick Break. I choose Ice Beam over Superpower because I needed to hit pokemon like Gliscor and Dragon pokemon hard, while Superpower deals with Dark, Normal and Steel pokemon. The only normal pokemon you see are Snorlax and Blissey, and Excadrill can take care of Steel, Rock and Dark pokemon, and also Chansey/Blissey. Snorlax is kind of annoying, but Will-O-Wispers and Reuniclus can bring it down at last, or Ferrothorn can Seed it.
Excadrill (M) - Air Balloon
Nature: Adamant
Trait: Sand Rush
EVs: 4 HP/252 Att/252 Spe
-Earthquake
-Rock Slide
-X-Scizzor
-Swords Dance
Yeah, Excadrill is pretty much standard in a Sandstorm team. I gave it a balloon so it can switch around easier in Earthquakes or if there are Spikes on the field, since I lack a spinner on my team. I gave it an Adamant nature for extra power since it doesn't need the extra Speed, since it already get's doubled by Sand Rush. With 252 Speed EVs and Sand Rush, you will be able to outspeed even Scarfed 130's, 252 Attack EVs is for raw power. Swords Dance is on the switch, and Earthquake is my main STAB attack. Rock Slide kills annoying flyers like Togekiss that would usually KO me with Aura Sphere. X-Scizzor kills pokemon like Hydreigon and Reuniclus after one or two Swords Dances.
Rotom Wash Form - Leftovers
Nature: Modest
Trait: Levitate
EVs: 124 HP/252 SAtk/132 Spe
-Hydro Pump
-Thunderbolt
-Will-O-Wisp
-Pain Split
Yeah, I love this Rotom. First I was using a bulky SD Scizor in my team, but it couldn't find its spot there that well. Its bulk, combined with its huge attack power can really opose a threat to other teams. Hydro Pump and ThunderBotl are it's main STAB attacks, and they give really great coverage. Will-O-Wisp is to spread status and cripples enemy's like Scizor and Dragonite. Pain Split is it's only healing move, but it works quite well on Rotom.
Ferrothorn (M) - Leftovers
Nature: Impish
Trait: Iron Barbs
EVs: 252 HP/48 Def/208 SDef
-Spikes
-Leech Seed
-Power Whip
-Thunder Wave
This is a great wall in Sandstorm, and it pairs well in terms of entry hazards by laying down Spikes after Hippo has done the same with Stealth Rocks. It would not be the first time I succeed in that, and when you did, it really kills ^^. Power Whip is it's STAB Attack and can do quite some damage on pokemon that are weak to it. Thunder Wave is to cripple fast sweepers, and Leech Seed aids in the outstalling of an enemy, while healing myself of course ^^.
Reuniclus (M) - Life Orb
Nature: Quiet
Trait: Magic Guard
EVs: 192 HP/64 Def/252 SAtk
IVs: 0 Speed
-Trick Room
-Calm Mind
-Psychic
-Recover
Kind of a standard Reuniclus set, but it works pretty awesome in this team. When this one used Trick Room, it outspeeds almost everything, and that together with great bulk and power can be really killing. Goal is to set up Trick Room and collect as many Calm Minds as possible. Recover is to heal any damage gained while collect CM's, and Psychic is it's Strongest STAB attack and can destroy after 2 CM's. Magi Guard is great, as it can switch into entry hazards without having to worry about damage. It also will not receive any damage from it's Life Orb, Sandstorm, Poison or Burn.
Heatran - Air Balloon
Nature: Calm
Trait: Flash Fire
EVs: 252 HP/4 Def/252 SpD
-Lava Plume
-Roar
-Toxic
-Protect
Well, sice I decided to replace Hippowdon with Tyranitar, I needed a new phazer. Heatran perfectly covers up that spot. It's great typing allows it to wall a lot, and with the popularity of pokemon like Scizor and Ferrothorn these days, it can switch around a lot, forcing switches and instantly Roaring them away for some additional SR damage. Toxic is to spread status around and to cripple any walls that do not have the ability to clear their status. Protect is to scout what a reaction will be or to Toxicstall. Lava Plume over Fire Blast because of the accuracy and PP, and over Flamethrower because of the 30% burn hax.
Pokemon already used:
Item: Lum Berry
EV's: 252 HP, 40 Atk, 216 SpD
Nature: Adamant
Ability: Technician
- Bullet Punch
- Bug Bite / Superpower / Pursuit
- Swords Dance
- Roost
Jellicent (M) - Leftovers
Nature: Bold
Trait: Water Absorb
EVs: 248 HP/216 Def/44 Spe
-Ice Beam
-Will-O-Wisp
-Taunt
-Recover
Hippowdon (M) - Leftovers
Nature: Impish
Trait: Sand Stream (duh).
EVs: 252 HP/252 Def/4 SDef
-Earthquake
-Stealth Rock
-Slack Off
-Roar
A quick look at the team:






Let's go in-depth

Tyranitar (M) - Leftovers
Trait: Sand Stream
Nature: Sassy
EVs: 252 HP/64 SAtk/192 SDef
A Sassy nature with 192 SDef EVs and the SDef boost from Sand Stream allows it to set up SR on almost any special lead. It packs quite a punch, with Crunch as its main STAB move to kill enemys like Reuniclus and Celebi. Fire Blast is for the frequent amount of Steel pokemon used as leads (Ferrothorn, Skarmory, Forretress), and can also kill Scizor if it doesn't has Superpower or Brick Break. I choose Ice Beam over Superpower because I needed to hit pokemon like Gliscor and Dragon pokemon hard, while Superpower deals with Dark, Normal and Steel pokemon. The only normal pokemon you see are Snorlax and Blissey, and Excadrill can take care of Steel, Rock and Dark pokemon, and also Chansey/Blissey. Snorlax is kind of annoying, but Will-O-Wispers and Reuniclus can bring it down at last, or Ferrothorn can Seed it.

Excadrill (M) - Air Balloon
Nature: Adamant
Trait: Sand Rush
EVs: 4 HP/252 Att/252 Spe
-Earthquake
-Rock Slide
-X-Scizzor
-Swords Dance
Yeah, Excadrill is pretty much standard in a Sandstorm team. I gave it a balloon so it can switch around easier in Earthquakes or if there are Spikes on the field, since I lack a spinner on my team. I gave it an Adamant nature for extra power since it doesn't need the extra Speed, since it already get's doubled by Sand Rush. With 252 Speed EVs and Sand Rush, you will be able to outspeed even Scarfed 130's, 252 Attack EVs is for raw power. Swords Dance is on the switch, and Earthquake is my main STAB attack. Rock Slide kills annoying flyers like Togekiss that would usually KO me with Aura Sphere. X-Scizzor kills pokemon like Hydreigon and Reuniclus after one or two Swords Dances.

Rotom Wash Form - Leftovers
Nature: Modest
Trait: Levitate
EVs: 124 HP/252 SAtk/132 Spe
-Hydro Pump
-Thunderbolt
-Will-O-Wisp
-Pain Split
Yeah, I love this Rotom. First I was using a bulky SD Scizor in my team, but it couldn't find its spot there that well. Its bulk, combined with its huge attack power can really opose a threat to other teams. Hydro Pump and ThunderBotl are it's main STAB attacks, and they give really great coverage. Will-O-Wisp is to spread status and cripples enemy's like Scizor and Dragonite. Pain Split is it's only healing move, but it works quite well on Rotom.

Ferrothorn (M) - Leftovers
Nature: Impish
Trait: Iron Barbs
EVs: 252 HP/48 Def/208 SDef
-Spikes
-Leech Seed
-Power Whip
-Thunder Wave
This is a great wall in Sandstorm, and it pairs well in terms of entry hazards by laying down Spikes after Hippo has done the same with Stealth Rocks. It would not be the first time I succeed in that, and when you did, it really kills ^^. Power Whip is it's STAB Attack and can do quite some damage on pokemon that are weak to it. Thunder Wave is to cripple fast sweepers, and Leech Seed aids in the outstalling of an enemy, while healing myself of course ^^.

Reuniclus (M) - Life Orb
Nature: Quiet
Trait: Magic Guard
EVs: 192 HP/64 Def/252 SAtk
IVs: 0 Speed
-Trick Room
-Calm Mind
-Psychic
-Recover
Kind of a standard Reuniclus set, but it works pretty awesome in this team. When this one used Trick Room, it outspeeds almost everything, and that together with great bulk and power can be really killing. Goal is to set up Trick Room and collect as many Calm Minds as possible. Recover is to heal any damage gained while collect CM's, and Psychic is it's Strongest STAB attack and can destroy after 2 CM's. Magi Guard is great, as it can switch into entry hazards without having to worry about damage. It also will not receive any damage from it's Life Orb, Sandstorm, Poison or Burn.

Heatran - Air Balloon
Nature: Calm
Trait: Flash Fire
EVs: 252 HP/4 Def/252 SpD
-Lava Plume
-Roar
-Toxic
-Protect
Well, sice I decided to replace Hippowdon with Tyranitar, I needed a new phazer. Heatran perfectly covers up that spot. It's great typing allows it to wall a lot, and with the popularity of pokemon like Scizor and Ferrothorn these days, it can switch around a lot, forcing switches and instantly Roaring them away for some additional SR damage. Toxic is to spread status around and to cripple any walls that do not have the ability to clear their status. Protect is to scout what a reaction will be or to Toxicstall. Lava Plume over Fire Blast because of the accuracy and PP, and over Flamethrower because of the 30% burn hax.
Pokemon already used:

Item: Lum Berry
EV's: 252 HP, 40 Atk, 216 SpD
Nature: Adamant
Ability: Technician
- Bullet Punch
- Bug Bite / Superpower / Pursuit
- Swords Dance
- Roost

Jellicent (M) - Leftovers
Nature: Bold
Trait: Water Absorb
EVs: 248 HP/216 Def/44 Spe
-Ice Beam
-Will-O-Wisp
-Taunt
-Recover

Hippowdon (M) - Leftovers
Nature: Impish
Trait: Sand Stream (duh).
EVs: 252 HP/252 Def/4 SDef
-Earthquake
-Stealth Rock
-Slack Off
-Roar