Because they're not broken, it's that simple. Smogon couldn't care less if a Pokemon is legendary or not. If a certain Pokemon does not centralize the tier, it stays. If it does, it gets out. As of right now, only one of the Chinese Quartet is considered by the majority of ou players as broken, Nemo, aka Chi Yu. Just about everybody in the smogon forums wants this goldfish to get banned. The problem is at the time of this answer, the terastal suspect is still ongoing and as such, no bans are happening until it's over. Once ths suspect its over, Nemo is probably gonna get quickbanned
The case for those three you mentioned are different. Flutter Mane is an absolutely ridiculous Pokemon to deal with. Ghost fairy provides just about perfect coverage and it even has mystical fire to melt steels. Combine that with having 135 in its spa and spe, yeah this thing was considered broken. It can do absolutely anything it wants and get away of it and with terastal, becomes really difficult to revenge kill. The same kinda goes for Bundle. This time, whether you've been naughty or not, santa is gonna punish you. It's speed tier is really good and combine that with good coverage in water and ice, just destroys nearly everything. The battle becomes a dangerous guessing game where if you predict right, you live but if not, you've been a very naughty child and gets punished. Basically, you're on the naughty list even if you've been nice and nobody likes a santa that puts everyone on the naughty list
Houndstone is the only one that's different. The only reason it got banned is because of the move last respect, which it's the only one that gets it at the moment. The way this move works is the more dead teammates the user has, the more its damage grows. Afaik, this move also gains more power if you revive the dead, like with revival blessing, and then sacrifice them once more to feed the move's damage. Though noboddy ever uses that. This move is also a ghost type which means unless you are immune to it, you're gonna die. And I do mean immune as even resists just die due to raw power. Often times people would use sand rush to get the speed they need and just send their enemies to the graveyard with this good doggie. However, outside of last respects, this thing ain't broken, the council said so themselves. There have been talks that upon Pokemon home's connectivity to the game, this thing might get unbanned as Basculegion also gets the move and in its place, the move will get banned instead. Until Basculegions starts swimming with ou, candle doggie stays out
Finally, when it comes to ubers, it is not based on usage stats at all. Only ou and below are based on usage. Ubers is where stuff considered too overwhelming for standard play, aka ou, get booted to. It doesn't matter if a Pokemon is utter garbage in ubers. It stays there if it's broken to suffer a painful and miserable existence as the ou playerbase wants it gone and none of the ubers players want to use it. A perfect example of this is Genesect, which gets quickbanned like, a week after its release into a new generation's ou only to suffer from unusability in ubers
