Did they just choose fleeting subjects, or does pop culture relevancy go by way too fast for a typical TV pipeline to catch up with?
Both; many of the subjects they chose to satirize were already past their prime (Trump's presidency was coming to an end already, among other political targets), but as already said, social media has made pop culture and current events move so much faster as to render it impossible to properly satirize anything.
The point of satire is to expose and ridicule the unacknowledged flaws of society, but now that everyone can loudly announce their opinions to the whole world on social media in an instant, most traditional satire gets beaten to the punch and ends up merely aping what has already been voiced online.