>>31299408Because it's gimmicky (the set, I mean). Competitive games revolve around switching and pivoting. You wall stuff with resistances and force switches with offensive presence. Sitting there and setting up Aqua Ring is just asking for a Tapu Koko to switch in and force you out.
As for Milotic itself, it's actually pretty good. Burns are common enough that you can rely on Marvel Scale to be somewhat consistent, and it only got better with the burn nerf. Milotic is a decent bulky offensive mon, good stats, Recover, etc.
It's just outclassed. It doesn't do enough to stand out in OU, where everything is a swiss army knife of role compression and top of their roles.
Like
>>31301334 said, run it as a bulky pivot.
