An interview with Philip Glass regarding The Reaping suggests reasons that the soundtrack may have been rejected (his expertise was in character films rather than action, and the direction of the film kept changing):
What was your musical approach to this film?
Oh it was quite different. These kinds of films are more about action
instead of character, if I can put it that way. With Notes on a
Scandal, you're talking about interpersonal dynamics - they're very
delicate, and profoundly dramatic and challenging and exhausting in a
way. With The Reaping and Candyman, for that matter, you're talking
about events which you can't really explain, and can't be explained by
character - they have nothing to do with character! You're talking
about music that works more with the surface of the action than with
the depths of the character. It's a very different way of working, and
I had to learn how to do it.
When I was doing more abstract theater pieces, there are similarities
with working on those and horror films. Wouldn't it be funny to
compare doing a Beckett play and doing a slasher film? But it's true!
You're working with more abstract ideas with these kinds of films. I'm
putting it the way I experience working on it, and I've found that
horror movies are more difficult to do. They're more challenging for
me, and I find them very interesting. So I enjoyed working on Taking
Lives. I liked the director, and I liked the performances and it was a
very interesting film to work on. I felt that was true of The Reaping
too. It was a long process because, again, it was a film where the
director (Stephen Hopkins) really began to understand the film as they
were making it. There were whole sections re-shot, people re-cast, and
things were being changed all along. It was a film that was a voyage
of discovery, there's no question.
Glass is known for self-plagiarizing, and I have found a review suggesting that he used some of the rejected material in his 2007 opera Appomattox, which might explain why we are not finding recordings of the score or copies of the film with that score.