Official 
post (which disavows Roger Craig Smith)
Seriously why. Sonic and the Black Knight was the last one and it was also the last time you could beat the final boss to a kickaas buttrock theme song. Why don't we have this anymore?
The Sonic Sound Team had rock songs for Sonic Frontiers like Undefeatable. Its not the same genre but it's the same vein.
Then Sonic x Shadow Generations had some remixes in there befitting the game if memory serves.
And here's where I learn that Un-gravitify is 19 years old jeez.
But they still kind of do. I don't think it's fair to say the rock soundtrack isn't a core part of the experience