Kevin Kiner scored the soundtrack for The Clone Wars and the vast majority of the music was recorded with a live orchestra performed by live musicians. Kiner also composes the score for Star Wars: Rebels but everything I've heard so far has been synthesized music, not performed by live musicians. (I've only seen season 1 so far).
The Star Wars franchise has usually shown a good appreciation for how music can elevate a project. Legendary composer John Williams has scored all the films thus far. They've even gotten live musicians to record brand new music not only for The Clone Wars TV show but also for video games like Star Wars: The Force Unleashed, The Old Republic, Kinect, and Battlefront.
Nearly everything else about Star Wars: Rebels has shown that its creators are willing to make the extra effort to improve the show like bringing back actors from the actual films to reprise their roles (James Earl Jones, Frank Oz, Billy Dee Williams, etc.) so why not cough up the money and hire some live musicians to record the music?
If they still care about having quality soundtracks (like I think they do) and the have the money to bring back actors from the films, then why does Star Wars: Rebels have a synthesized soundtrack?