I don't think there are more mtn bike clinics. It's just that they're easier to find. There will often be mtn bike clinics associated with larger races. And most mtn bike clinics are marketed to the more recreational riders (not the racers).
For road clinics, there are TONS all over the country. However, most are advertised through race-oriented organizations (even if they're not just race clinics). Check the website of local coaches or coaching organizations or clubs. Ask at your LBS.
Most recreational road riders don't understand the importance of learning skills, while most recreational mtn bikers do it because they realize that they can't improve unless they do (learn skills).
BTW, I bet Jeremy never took a mtn bike or bmx clinic. But you mentioned that he learned his skills as a kid. That's a very telling statement. Children are much more natural learners when it comes to anything that involves risk (like cycling). He attributes his skills to riding mtb & bmx, but the reality of it is that if he'd had a road bike as a kid, he'd probably learn the same skills. Kids are fabulous learners.