Yo Boost
To help answer your ?
I think since we are getting into ep 5, and moving away from the ep3/ep4 midpoint, that the prequel's music is no longer necessary, Ya know what I mean?
I guess What I'm trying to say is that it's a new era. New era= different music
And The reason why Ady put Battle of the Heros in for the Ben/Vader fight is because it was the rematch, or a continuation of the fight on Mustafar.
...I suck at explaining things =P