I like riding on the road because I can cover long distances fairly easily and feel like I have achieved something, 80kms in 3 hours of a morning is a nice feeling when the same distance off road is an epic and requires swmbo approval.
Also, aside from Ashton Court and Leigh Woods, I have to ride 10 miles out and back on roads from home to get to some good bridleways and rupps so its a bit of a pain….. and cleaning, even with a hardtail its just an utter ball ache at this time of year….
I still love it though, when you are riding hard on a road bike, you are invariably just trying to maintain a high speed or climbing a steep hill which are staisfying in their own way but do get a bit boring. Riding hard on an mtb has far more variables to manage combining rapid direction change, technical obstacles (jumps, berms etc), loose surfaces and varying grip as well as the danger element of trees and things in the close vicinity so there is lots more “to do” for the rider so to speak which is why, given the chance, I’d ride mtb more than I currently do.