Well season 1 of Black and White kept most of the original music, even after the first 5 episodes which just kept everything without any dub added cues.
After that, what the Muk happened?
I guess we'll never know... I wish we could at least get an official answer about it (although even if we were to get one they'd probably say some bs thing to avoid properly explaining the real reasons)
I do think some of the new dub bgm introduced in XY is actually kind of nice... but obviously not even close to the original bgm (it really hurts me the most when we lose game music - part of the point of the anime is the bring the games to life, and it is so exciting to hear the game music in the show - why is it acceptable to have this music in the English version of the games yet so rare that they allow us to hear it in the English version of the show? ugh)