My big local facebook bike group is 90% road bikes. I think a lot of it has to do with ease of getting started/fear of getting hurt. Anybody can get a road bike and head out their front door and ride. Mtn biking requires you to be able to get to a trail head and for some reason people think it is more scary than having cars flying 2ft from you at 50mph
. When I do a road ride they all seem surprised when I say road riding scares me a lot more than mtn biking. This has been a bad year for my local group. Seems like somebody has been hit by a car every month and we have had multiple dog bites. It just doesn't do it for me but when it is raining all the time I have to get a ride in some how. I hate riding with cars and the idea of sliding on asphalt doesn't appeal to me. Road biking is great exercise since you can just pick a level of effort and sustain it for long periods of time. Mtn biking is more about fun IMO, your effort has to vary a lot just due to the terrain.