Why doesn't south park ever get sued?

Because Jerry Falwell sued Hustler magazine and lost. That set the precedent that you can take the piss out of public figures all you want in the US. 

For what? It's parody...

They write it in smart, specific ways so the wording isn't untruthful or unfair, and so they can't be sued so much, or when they are they can win.

It is amazing that the SJW haven't come after them to cancel them and somehow they've stayed under the radar.

It has been tried and thrown out of court.

Most of what they do is protected under fair use/parody laws. They’ve been sued a few times for people claiming they stole similar cartoon ideas from them but South Park won in all three cases. People say that they were sued by Yelp, but apparently that was a rumor made up by a site that regularly says untrue things. 

Yelp is suing the creators (Parker & Stone) for $10 million. 

Satire is protected free speech.

If you were specific about what you think they should get sue for, it might help.