Based on what I see here, I would have to say a good amount of it is the variety of quality and prices available in used MTBs. You can find about anything you want in that area, while road bikes are either expensive or junky it seems. not much in the middle.

Rack mounts are more common on MTBs it seems and, as you said, the flat handlebars seem to be better in traffic. I'm sure a lot of it has to do with image or being intimidated by road bikes. Also, MTBs work better on the road than road bikes do off. If you can only have one bike but want to do both it makes sense.

Toughness...I'm sure most have their good reasons. I'm also sure some really wouldn't have an answer, it's just what they use.