I think some of it is that the "political element" of the past became mainstream. Partly because it was a political element once.

People now, who most likely watched Trek in reruns 20 years after the fact don't see Uhura or Chekov or the Black Flag Admiral as political, because it no lomger really was when they watched.

But then they see a gay character on modern Trek and flip out. But others just see, another character. And in 20 years, maybe no one will even think it is or realize it was polotical.

That sort of thing.

Even on your futurama example this applies to some extent, though we do kimd of still have a garbage problem.