Well to be fair, it didn't happen overnight in the movie, it happened gradually as people became more stupid. I've never had a chance to write down my analysis of this so I'm gonna do it now. This is a very subtle and yet hilarious commentary on society and stupidity.

As people continue to decline into stupidity, the first change was from Fuddruckers to Futtbuckers. This was simply more directly crude. Fuddruckers has always had the common reputation (true or not, doesn't matter) that its name was a sly spoonerism of Ruddfuckers. But since Rudd isn't a word, the first change to Futtbuckers makes sense, from a stupid person's point of view. Now the title is a far less sly spoonerism, as the change itself makes it obvious what they were shooting for: a spoonerism of Buttfuckers. So while humanity is stupider, it is still capable of somewhat intricate humor.

As people continue to get more stupid, they can't even comprehend what a spoonerism is, so the next change is from Futtbuckers to Buttruckers, because to a badly stupid person, Futtbuckers is a nonsense word - the intricacy in the humor is now wholly lost - but futt sounds like butt and butt is hilarious, so why not put it in the name? They have no comprehension why the original ruckers was changed to buckers, so they revert to the original ruckers.

Alas, people get even more stupid and at this level of stupidity, Buttruckers now seems unnecessarily polite. If butt is hilarious, the fuck is even more hilarious and ruckers is a nonsense word so why not use fuckers? So they call it Buttfuckers and they find it hilarious but the fact that it's wildly offensive and is, ultimately, the pinnacle of the dirty humor only vaguely suggested by the original Fuddruckers (and directly suggested by the subsequent Futtbuckers), completely escapes them.

As people get even stupider the simple fact that Buttfuckers is in any way inappropriate is now completely lost, as the place is shown as popular for hosting children's parties. It's not even a funny joke anymore because the people are too stupid to even recognize the word.

TL;DR: Fuddruckers -> Futtbuckers -> Buttruckers -> Buttfuckers