True, Jun Senoue is more than capable of composing some good tracks, and has done for sonic games since the Megadrive. Sonic Mania also had some great tracks.
I don't believe that Sega has publicly said why, but its believed to be either Sega not owning the rights to the music, as it was co-written by Michael Jackson, and not wanting to be associated with him after he went off the deep end in the 90s. I believe there are other games in collections which are omitted despite popular demand, as they are rights on songs contained within are on dodgy ground so are omitted to avoid legal issues.
First reason would be idiotic. I assume they lost licensing rights. I'm sure there's a mod for the PC version I could download if I really wanted the music. ---