Silent For Too Long said:
Trey Parker and Matt Stone were glaringly obvious in their refusal to make fun of Biden, which sucks to say as generally a big fan of their work.
I'm pretty sure he is the only President in the history of their show they almost completely ignored.
Money talks.
They got a massive deal with Paramount.
Fallon was mostly silly with his political humor until he played with Trump's hair and his network got offended.
Most of them are politically to the left, but I think industry money and pressure influence their moves more than anything else.
Leno was wise to get out when he did. He wasn't conservative, but he would have been portrayed to be in the current climate.