The show was great until about season 8, then it gradually declined until about season 12, then it just became another laugh-track sitcom and plummeted into Zombie Simpsons, which is what we have today.

Contributing factors:
- The three creators were all gone by season 8.
- The writing staff had about 50% overturn by season 8, and pretty much everyone was gone by season 12.
- The show was originally counter-culture to the traditional family sitcom, which eventually went out of style, so it didn't really have anything that it was counter-culture to anymore.
- The staff used to put a lot more effort into the script. It went from like 30-40 script rewrites per episode to being a somewhat low-effort formula that they just push out episode to episode.
- And a bunch of other stuff, but those are the highlights.