I know this has been asked for but I don't think we've got a reasoning as to why swampert is in B rather than B+ or even A-. Its a fantastic wall that can utilize slow momentum with flip turn, and helps check a lot of mons like excadrill, blaziken, dracozolt, landorus-t, and more. It's definitely just as good as something like hydriegon or dracozolt in this metagame, if not even better.
It faces heavy competition from loads of mons, such as Lando as the team's ground type, pex as a team's bulky water type, and the plethora of excellent rockers in the tier.
Offensively it's alright with a band. It's one of the stronger offensive water types that can function without rain and it has good coverage. But it's still slow and other mons can make use of a choice band much more effectively (mosa, shifu, boom).
Defensively it's eh. It lacks recovery making it more susceptible to hazard and status damage. Because of this it's also forced to run leftovers meaning it can't use rocky helmet like many other physical walls in the current meta.
Phys def can check melmetal, cinderace, lando, drill, blaziken and dragonite which is great. But you're still 2hkod by mosa and shifu. And once you're chipped you get 2hkod by everything.
Spdef can take a hit from mons like spectrier, nidoking and latios, but pert isn't recovering that health back. So when they come back for round 2 pert's gonna die and you're gonna be sat there wishing you brought a better specially defensive water type. Oh and btw it gets 2hkod by specs zap hurricane.
Also, you'd expect your ground type to be checking your opponent's electric type, but it always gets 2hkod by specs koko grass knot, and it can't touch zap without ice punch or stone edge, which only feature on AV and band sets.
As a defensive pivot you have the excellent slowtwins who, have recovery, regenerator, boots, future sight and teleport. And they check almost all of the same mons that pert checks and more.
There are cases where pert will be the only mon that can fulfil a team's needs. Being a ground type, rocker, bulky water and defensive pivot all in one gives you more freedom when building. However, pert's flaws are bad enough to the point where they make it worse than almost every other alternative in most cases.
tl:dr has a niche, good role compression, generally outclassed due to its lack of recovery and offensive options (that don't compromise its defensive capabilities).