Hans Zimmer says in an interview to Inverse that he felt great loyalty to Christian Bale's portrayal of Batman and scoring for the new Batman would feel like betrayal.
He tried to come up with something for Ben but could not because Ben plays it differently than Christian. According to Zimmer, Ben's character is more middle-aged and "grumpy" as hell but Zimmer couldn't "feel the pain" that he felt in Christian's performance and it was that pain which interested him enough to score for the movie.
LMFAO wtf kind of pain did he sense in Christian's generic-ass performance?? Zimmer is so self-important and serious.