As previously stated, the Illuminights soundtrack is being offered because the music is original and was produced specifically for the event. Much of the other music used at Busch Gardens is licensed only for them to play, but not to sell. Attempting to compile an album of their music would require contacting various sources and paying more fees, thus the park currently does not deem it a viable item to sell.
Note that even music used in Busch Gardens' productions is not original. The Jack is Back soundtrack includes "Dancing Queen" which of course was not produced by BG. Celtic Fyre's first song "Streets of Dublin" is from the musical A Man of No Importance. Although the orchestration was done for Busch, the song is licensed for use.
For HOS, much of the pathway soundtrack is by Midnight Syndicate.
I know particularly for HOS that the music in San Marco is Graveyard by Midnight Syndicate, the music at Pompeii is Ashielf Pi by EH Posthumus, and the music in England when it was Piratical Purgatory was Harbored Souls by Midnight Syndicate.
For Christmas Town, a few tracks are from the Polar Express soundtrack.