I think it is more than poor promotion or fan interest. Motorcycle road racing has become too expensive for laymen. As soon as riders hit the 'expert' level, their costs go parabolic if they want to be competitive. Not too many families will afford the $50k/year it takes to race a limited club schedule with a couple pro events thrown in.
Add in the fact that there are no real purses anymore, few cash contingency pots to shoot for, no media coverage to sell; and the only people left to populate the grids are the same old guys that have been there for decades, and a handful of new riders, supported by families that haven't run out of budget yet.