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?
To be fair, that was also the general consensus when the games were actually new, seemed like right around Sonic Heroes a lot of people started having misgivings about the games. It took the ten-year-olds who played it coming of age for the consensus to shift, which makes sense. Been interesting to watch.
I should also note however that Sonic CD had two vocal themes, both of which slap, so y'know.