It's about the capsule you're in. Mountain biking was HUGE in the early-mid 90's, then crashed hard. It may or may not be coming back among road racers who are looking for new competitive spheres *cough* but I'm not so sure that it's ready to make road racing a cross-training activity. But as it continues to be very difficult to organize a road race in many places, off-road racing might have an opportunity there... not that it's necessarily easy to get a cross race up and running, either, but not needing to close roads certainly helps.