WAG here, but it might have to do with the fact that while the 9mm, and .40 use the same frame (the USP was originally designed as a .40 gun), the .45 uses a slightly larger one.
Additionally, with the magazine caps, .45's are likely to be more popular than the .40's, and 9mm which are affected by the magazine ban, whereas the compact .45 is not.
Again, just a WAG here. But that would be my reasoning.
-Morgan