Games on rails that resemble a racing game in their layout and gameplay are easier to think of, harder to fail at them and harder for the player to get even into some gameplay problem. Also if i remember correctly it was a matter of that they couldn't get proper 3D gameplay done, after the failure with overambitious Sonic X-Treme and the experiment with Sonic
World so went for the easier way (not that it worked well, given the amount of bugs the first one had (Sonic 2006)). And with the fanbase for the new ones they have now, would be silly to go back.
Kinn, if you want that kind of Sonic gameplay, go for games that go out of SAGE's
or out of Sonic Hacking Contest's
. There is some very good ones out of there. Fun fact is that one of the games that out out of there was done in a Doom engine (Doom Legacy), and if i remember well was quite good in my eyes, at least in adapting Sonic's gameplay into full3D and layout wise.
An example of fangame of that kind.
the one made on a Doom engine.