I've always wondered why they didn't as well. If you lay an M9 down next to a full sized USP 45 they're about the same size. I'm sure they could build a 10-12 shot 45 in the same sized package. Even though I think long term a Glock 21 or FNP-45 would be a better pick a 45 version of the M9 would be an easy transition with no retraining needed. It shouldn't be about compromise but that would be a decent choice.