Maybe it might be following the curse like The Simpsons, much as I love Simpsons up until season nine. When you've been on the air for so long, you start to get a little stale.
I think for me, season twelve might have been the wake up call since there were only about three or four episodes that I've liked:
+Britney's New Look
+Major Boobage
+Breast Cancer Show Ever
The Canada On Strike episode came mildly close due to the amusing song.
I didn't like Season 13 at all.