The only place where mountain bikers are excluded, are these wilderness trails we're talking about. The density of riders around population centers is simply a reflection of the mountain bike lifestyle. Because many riders hit the trails almost daily, they usually ride close to home, and most trail systems are close to most population centers. That's where the users are.
What we've been seeing as of late in the MTB community is a greater scrutiny of which designated Wilderness areas deserve that protection. Some wilderness areas seem to have earned their designation somewhat randomly, or at the least the boundaries seem randomly drawn.
As others have said, the biggest advantage the MTB community has in their advocacy arsenal is the low impact nature of the activity. Second to that is the willingness of the community to assemble in mass to grab shovels and repair or build their own trails. I also see within our own regional advocacy groups, a good bit of compromise. The MTB groups here are very strategic, willing to give up a trail here, to gain a better one there.
Not to keep hammering on the 4x4 advocacy groups, but they really just don't get it, at least not as a whole. They pick battles they can't win, war over turf they really shouldn't defend, and most importantly, don't realize that they are a minority advocacy group and made even smaller by divisiveness.
There's the kicker. It's not entirely about the size of the user group, but the size of the advocacy voice within that group.