Because they're a PART of the thing they used to mock. South Park has become mainstream.

This isn't even a particularly modern phenomenon. It happened to the "rebellious" Reynard cycle in the Middle Ages. What started out as funny animal stories meant to mock the elites (even Reynard himself wasn't a peasant, but a low level noble with his own estate) eventually started to be told and recirculated by the elites themselves as the later Reynard/Reynardine stories. The difference? Whereas in the original stories Reynard would get away with his shenanigans more often than not (usually successfully blaming the bishop Isengrim and getting him hanged in his stead), Reynardine (son of Reynard) would often be brought to justice.

One can see the same shift in modern Looney Tunes vs the original theatrical shorts; they seem soulless to me, like they're following what they think is the old formula, but that's all they're doing. The originals were as good as they were because the animators had no oversight, and Jack Warner didn't give two fucks about cartoons as long as they made money.

What was once wild, gets tamed if the elites can't (or don't want to) kill it.