Maybe the majority of MTB riders are restricted to the many walking/bike trails in and around urban areas. I would not be surprised.I have the same anecdotes. When in the backcountry and remote areas, I don't see any MTB riders.
But on trails within 10 miles of an urban area, I cannot count all the MTB riders. They are everywhere.
So they clearly outnumber the 4x4 crowd and have better advocacy groups, funding, demographics, etc....
I belong to the MTB, OHV and 4x4 user groups. And based on my experience, the MTB advocacy groups walk circles around the others.
All that said, I do not know whether or not I support MTB access in designated wilderness areas.
As to MTB access in designated Wilderness - As I see it, there are two types of Wilderness: (1) that conforming to the original intent of the Act, and (2) all the rest since Wilderness (in all its incarnations) mission creep set in. Keep the original Wilderness areas bipedal transport only. I'd be pleased if all the rest were returned to plain old FS and BLM status.
Hard for me to look at a piece of ground and think wilderness when it obviously has been; mined, logged, homesteaded, fenced, railroaded and is crisscrossed with roads.