@Mrpetuski makes some good points in comparison to Scott Malkinson and Butters, who have been appearing fairly frequently, but I would add, often end up imitating Cartman and exhibiting sadism and misogyny independently of him, too; which, again, is something Pip does not have on his record anywhere.
I think a really important factor in his favor is that because Pip was retired so early in the show's run, he is relatively innocent and unscathed, whereas it's common to see users to fixate on whenever Stan/Kyle/Butters/Kenny/etc has done something wrong. Pip has no such 'baggage' and his story is finished. He is one of the very few characters in the 'South Park' canon who hasn't really, at any point, done something wrong, which is balanced by the fact the character was often bullied, was retired and eventually killed off, having done nothing to hint at deserving it, in a show where Cartman escapes consequences quite often.
I'm a fandom vet here and before "201" was one of the people who campaigned for Pip to have a role again; my interest in him dwindled off a lot after that for obvious reasons, so I've sort of been on both sides of this. He's had a big uptick in interest in the last couple years because of a fan comic called "Hell Park" in which he plays a key role, might be worth mentioning.