It's because the sonic franchise has been constantly attempting to break into the 3d world and no attempt has been successful in capturing what made Sonic FUN.
Basically, the problem stems from the fact that the 3d world is just too complicated to be able to set up in the rollercoaster style paths. the 2d world also allowed you the ability to identify paths much faster since your options were so much more limited.
Really, when it comes down to it, the original 2D sonic worlds were a bunch of slides and ramps, springs and rollercoaster tracks. The only reason it didn't look bad was cause it was a 2D world. 3D makes doing that look awful, but trying to do a non rollercoaster looking world screws up the ability to kick in any real sonic speed to the game.
So basically, the best 3d game for sonic was the original one on the dreamcast, since then it's become a focus for a new character to be introduced and subsequently used and milked for the franchise to every extent possible, when what people really want is Sonic, and fast gameplay. Heck, the first one still ended up with you playing through as like 8 different characters
holy crap, when I started my reply, there were still no replies to this thing, geeze people.
Also, I wonder if Sonic would do better to do the 2.5D that nintendo used for Kirby.