Its like this.
Sega and Sonic Team really just don't want Sonic to be the same formula over and over. Sure if it ain't broke don't fix sounds cool, I mean Nintendo has been doing it for years and the games are still good. I think the problem is not that they want every Sonic game to be different and fresh, I think its simply that they try to change too much stuff. Yet Sonic fans (such as myself) still get the games and find enjoyment out of them despite the scores.

So really what ST has been trying to do for years is find the perfect balance of old and new and they are slowly getting there (Sonic Rush, Sonic Rush Adventure, Unleashed, Colors).