It's because Simpsons has grown up with it's creators. No longer are the hip, youthful, topical references used in the show because the creative team is, i'm sad to say, hip, youthful, or topical.
It's the evolution of any Primetime series. Just because it's animated and the actors don't get any older, so to speak, doesn't mean the creative spark of what was once a great show can stay bright and flaming.
The Simpsons paved the way for for Primetime cartoons. Talk about setting the standard. If there were no Simpsons there would be no Family Guy or South Park or Futurama or American Dad or King of the Hill. Even Beavis and Butthead (back in the day) had a rapt audience of angry Gen X-ers because the creators knew that the formula already worked thanks to Matt Groening and Tracy Ullman.
I haven't been a big fan of the Simpsons, beyond the purposes of nostalgia, for a while, but I will always be grateful for the road the show paved almost twenty five years ago.