Why do people think the new simpsons episodes are bad? Because they don't like them. Personally, having not watched Simpsons out of an urgent sense of not wanting to watch any more cringe-worthy bad TV for several years now (this is almost all TV, not just the Simpsons) I just think it's boring. The characters are played out, and have been since season 6 or so. The episodes are formulaic, preachy and generally uninteresting, the political commentary is weak and colourless, the jokes are not funny and the entire show just seems to be resting on four typecast characters going through the same motions every episode.
I don't blame people who find it funny. These are purely my own thoughts on the show. Comparing it to what I watch, it has far less likable characters than Daria, far less humour than Freakazoid or Drawn Together, the political commentary is better in the Daily Show and engaging storylines are better found in Doctor Who.
Personally, however, I dislike almost anything in the category Matt Groening and Seth McFarlane. Simpsons, Futurama, Family Guy, American Dad.. trite and boring in my opinion. And yes, I've seen enough of it to know what I'm talking about.