I'd say first of all the 9mm is easily the most popular handgun caliber these days, so a lot of folks I'm sure like having multiple guns in the same caliber so they don't have to collect different calibers. AS has been pointed out, it's also the cheapest to shoot.

As for the BHP vs 1911, I have a FNHP that I love, but it's still no 1911. The trigger is not as good, and the safety lever is not is well designed as on the 1911. My thumb finds the 1911 safety without trying, but the HP I have to hunt for it a bit.