Because being part Psychic-type really sucks defensively (makes you neutral to Bug and Dark); that plus Mega Garde's horrible HP stat make it so that Sylveon has a much easier time coming in on many threats. Pure Fairy-type is actually very good defensively; also there's an opportunity cost for using Mega Gardevoir being the Mega slot. Psyshock and Shadow Ball are filler anyways, usually you're just clicking Hyper Voice or HP Fire if they have a Ferrothorn/Scizor. And yeah Sylveon does hit quite a bit harder too. The main factor is that Sylveon can actually switch in on stuff, while Gardevoir has to do things like predicting Secret Sword from Keldeo to get in safely.
On that note Magic Guard does not actually compensate for Clefable's worse special bulk...
252 SpA Life Orb Greninja Hydro Pump vs. 252 HP / 0+ SpD Sylveon: 144-172 (36.5 - 43.6%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery (Calm over Bold because it's more EV efficient if you're wondering)
252 SpA Life Orb Greninja Hydro Pump vs. 252 HP / 0 SpD Clefable: 218-257 (55.3 - 65.2%) -- guaranteed 2HKO after Leftovers recovery
First of all Stealth Rock needs to be up for that argument to even be made which is not always the case. Secondly, that's a significant difference in special bulk. Even if you factor in SR Sylveon still does better. Not to mention that Sylveon benefits greatly from a teammate like Defog Mandibuzz or Skarmory because they're physical walls, appreciate a switch in to Thundurus/Greninja and cleric support... You essentially get walls with reliable recovery for both sides of the spectrum, Defog, Wish, Heal Bell and decent offensive presence with just two team slots, yeah I'd sign up for that.
Either way, I feel like this Sylveon discussion is dragging on way longer than it needs to... I see people just reiterating the same arguments at this point. I suggest it moves up to B-, because it's definitely a legitimate mon in the current metagame (name something that reliably walls both Greninja and Thundurus-I while still having offensive presence for example) and it has a good niche over its competitors no matter whether it runs Specs or a Cleric set.
That's because you are comparing the wrong Clefable spread. With the right investment, Clefable can avoid the 2HKO from LO Thundurus's Thunderbolt and Greninja's LO Hydro Pump while having having better physical bulk than max HP / max Def+ Sylveon, factoring in Stealth Rock.
- 252 SpA Life Orb Greninja Hydro Pump vs. 252 HP / 96+ SpD Clefable: 177-211 (44.9 - 53.5%) -- 1.2% chance to 2HKO after Leftovers recovery
- 252 SpA Life Orb Greninja Hydro Pump vs. 252 HP / 0 SpD Sylveon: 160-188 (40.6 - 47.7%) -- 5.9% chance to 2HKO after Stealth Rock and Leftovers recovery
- 252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 96+ SpD Clefable: 165-196 (41.8 - 49.7%) -- guaranteed 3HKO after Leftovers recovery
- 252 SpA Life Orb Thundurus Thunderbolt vs. 252 HP / 0 SpD Sylveon: 149-177 (37.8 - 44.9%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
- 252 SpA Life Orb Latios Psyshock vs. 252 HP / 160 Def Clefable: 181-214 (45.9 - 54.3%) -- 5.5% chance to 2HKO after Leftovers recovery
- 252 SpA Life Orb Latios Psyshock vs. 252 HP / 252+ Def Sylveon: 161-191 (40.8 - 48.4%) -- 10.9% chance to 2HKO after Stealth Rock and Leftovers recovery
So, with the right spread and SR up, Clefable has a tad better overall bulk. Take into account residual damage from Pokemon those clerics are supposed to check, such as WoW Rotom-W, Scald Scarf Keldeo, and Scald / Toxic Slowbro, and you will find that Clefable has better walling abilities in the long run thanks to Magic Guard.
Regarding Doublade, my concern about moving it up is that's its very passive and lacks any kind of reliably recovery, making it very easy to wear down. It also lacks any kind of pivoting or support moves, so it provides zero team support. It checks a lot true, but it needs excessive team support to be worth it, and it's very passive. If enough people want it to go to C+ i will consider it, but C seems better for it.
Finally, i haven't seen a single good DD Salamence or Scarf Salamence team and i still think it's terribly outclassed. As a scarfer it lacks a spammable move, unlike popular scarfers such as Keldeo, Landorus-T, and Terrakion, is SR weak, and can't revenge kill dangerous sweepers such as Mega Gyarados and Mega Tyranitar from full health. It's supposed to be a late-game cleaner, but with many teams packing both a Steel-type, a Fairy-type, and priority, cleaning up with it is almost impossible without providing excessive team support, team support that would be better spend on something else. Dragon Dance has too much competition and is almost entirely outclassed by the plethora of Dragon Dance users available, either discounting Mega Evolutions. Dragon Dance Dragonite has Extremespeed and Multiscale, both of which make up for Mence's higher Speed and allow Dragonite to be a much bigger threat to offensive teams, being able to outspeed opposing priority and Scarf users after a DD boost. Even if Dragonite doesn't manage to set up, it can still be usefull thanks to Extremespeed, by revenge killing weakened offensive Pokemon, which can't be said about DD Salamence, which is easy to wall and doesn't really check anything. DD Gyarados has a way better defensive typing and overall bulk, which provide it more set up chances and make it harder to revenge kill, as it is not weak to any form of priority and resists Bullet Punch. It also has way better coverage with just Waterfall + Bounce and can use Substitute to avoid status.
tl;dr Salmence is severely outclassed and i don't see it moving out of D rank.