A lot of good responses. One thing that is not mentioned is image. Many people think road bikes are not manly and prefer to be riding something tougher looking. It's similar to the same reason SUVs are popular for single occupant suburban commutes.
I love all my bikes, but mountain bikes on the hilly, well paved roads around here suck. I commute on several different road bikes, and prefer it that way. With well built wheels, I've never had a problem with branches, curbs, potholes, etc. Low spoke racing wheels aren't appropriate, but that should be obvious.
But then again, I think SUVs are pretty stupid too unless you enjoy serious off roading.
Az