You pretty much just answered your own question there: LC has it's own rules, and whatever gets banned gets banned, be it the one ability or the whole Pokemon. Vulpix probably had sky-high usage when it was unbanned and it might have actually been pretty good (it can hit 17 Speed or 17 Sp. Def with correct investment). Meanwhile, Bidoof is just... not even worthy of being considered.
Bans in LC are used for things that are too gamebreaking. Ex: Speed Boost Carvanha, which literally could 6-0 teams on its own after one boost. Bidoof could never do that if it wanted to.