For more orchestral music, one would have to pay re-use fees to the members of the orchestra. However, for the stuff that the composers perform themselves it would cost significantly less...but there's still the question of widespread appeal. Of course, we'd love releases of this music, but a subsection of fans hardly spells profit for the studios whose products the music was written for.
Still, there can be exceptions. Many years ago, there was a promo CD for Kevin Manthei's "Invader Zim" music. It is still some of the finest music I've ever heard for an animated series. (His "Xiaolin Showdown" music is pretty good, too.)
In terms of favorites, Adam Berry's "Kim Possible" scoring is quite enjoyable. Christopher Tyng has written some exciting music for "Futurama" and, discovering the show for the first time last year, I found that "Galaxy High" had some terrific underscoring, courtesy of Don Felder.
With the movie, TV and song parodies that were packed into the music, Charlie Brissette's work on "The Angry Beavers" holds a place in my heart...and hopefully, one day, my collection.