Considering how dynamic the gameplay is, it wouldn't surprise me if they did that due to dynamic music. I never really noticed while my girlfriend was playing it. But it really wouldn't surprise me if the game utilizes dynamic music, which would be hard to put on a soundtrack, because then which version is the real / canon version?

But that's just me surmising for this one instance. I've had to deal with games I love having limited soundtrack releases too. Then having to scour for all the worthwhile tracks that were neglected