Many people are intimidated by road bikes and perceive them as twitchy and dangerous. A mountain bike seems familiar and sturdy. Yes, a hybrid would do just as well, but many people just don't get the difference between them. They see every flat bar bike as "mountain bike" and every drop bar bike as "racing bike". I've heard people refer to my touring bike as a "racer".
In the rack where I work, it's mostly cheap department store "mountain" bikes, a few conventional hybrids, two or three internally geared hybrids (not including mine), one fixie, and a few old rigid mtbs that were reasonably good quality in their day. Once in a while I'll take the touring bike, and was surprised one day to see another of the exact same model in the rack when I arrived. I'm always surprised when I see a road bike of any kind there, although a lot of 'cross bikes are sold and used here due to the lousy roads.