I'm kind of wondering the same thing myself now! I guess I tend to think of the B flat trumpet as the "default" in a brass ensemble, so when I started writing it, I started with two B flat trumpets. The first trumpet part, I realized, was going quite high, so I changed it to a D trumpet. But it would have made more sense to change the second to C trumpet for a more reasonable key signature. To be perfectly honest, I was rushing to try to finish this before Christmas, and didn't think it through properly.
Thanks for listening, and happy new year to you too!