SM OU RMT.. haven't played since Gen 3

[ABC-Trainer]

Hey I bought my first pokemon since FR/LG and I need some help. Trying to get a handle on how the metagame has changed since then.

Tapu Bulu @ Choice Band
Ability: Grassy Surge
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Wood Hammer
- Horn Leech
- Megahorn
- Superpower

I picked Tapu Bulu as an offensive core who can dish out damage for my team. His grass typing and ability provides a ground resist and healing for my team respectively. My team has a pretty bad ground weakness otherwise. I picked choice band set because the rest of my team lacks offense so I want something that can dish out damage.

Nihilego @ Focus Sash
Ability: Beast Boost
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Stealth Rock
- Toxic Spikes
- Power Gem
- Sludge Bomb

Nihilego is my favorite UB and I picked them as a suicide lead. I chose Toxic Spikes on this guy and not on Toxapex but I might consider throwing Toxic Spikes on Toxapex in lieu of Toxic Spikes here, but it's worked out ok so far. If for some reason this poke doesn't get KOd at first it can also prove to be a late game sweeper, as its EVs ensure that SpAtk gets boosted instead of Speed.

Komala @ Leftovers
Ability: Comatose
EVs: 252 HP / 4 Atk / 252 SpD
Calm Nature
- Rapid Spin
- Wish
- U-turn
- Earthquake

I picked Komala because it's cute and for utility. Rapid Spin, Wish and U-turn are all great utility moves. Picked the SpDef EVs so I can switch it into stuff. EQ for my steel weakness.

Toxapex @ Black Sludge
Ability: Regenerator
EVs: 252 HP / 4 Def / 252 SpD
Bold Nature
IVs: 0 Atk
- Haze
- Recover
- Scald
- Toxic

Toxapex is one of the best new pokes in this gen and I really liked playing it ingame so I chose it for my team. I picked Haze to counter stat boost pokemon. I'm thinking maybe Rocky Helmet might be better than Black Sludge because I already have regenerator but I'm not sure. Picked Toxic because Nihilego already has Toxic Spikes but I might switch off.

Tyranitar @ Rocky Helmet
Ability: Sand Stream
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Dragon Dance
- Stone Edge
- Fire Punch
- Crunch

Since half my pokemon have a psychic weak I picked TTar as a Psychic resist. I would prefer to have Tyranitarite but apparently Showdown doesn't let me choose that as an item. I picked Fire Punch over Ice Punch in order to counter Steel types. I think Rocky Helmet was a good choice as item as Tyranitarite is not available, but maybe someone can pick a better option?

Weezing @ Black Sludge
Ability: Levitate
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Sludge Bomb
- Will-O-Wisp
- Pain Split
- Fire Blast

I have a slight ground type weak so I picked Weezing also because he's so cute. Will-O-Wisp to burn offensive leads and Fire Blast to damage steel types. Apparently Weezing is really bad since Gen 3 but I think I can make him work although suggestions on his moveset would be appreciated.

I pretty much picked some pokes I liked.. picked my fave Tapu and UB, then Toxapex and Komala I liked a lot so I picked them as support. I picked TTar to cover my psychic weakness. I think I lose to steel but I put Fire Blast and Fire Punch on Weezing / TTar for that reason.

Can anyone explain why Tyranitarite is illegal even in Pokebank OU?