I think to sum it up, limited use. The 1/12 scale class was already the established limited use car, and so any other class/car had to bring something 1/12 scale didn't. This is why I feel TC is still popular. Just think of how many different ways 1/10th TC can be run:
Indoor Carpet Foam
Indoor Carpet Rubber
Indoor Asphault Rubber
Outdoor Asphault Rubber
....
and more importantly
....
BASHING.....
I feel that if 1/10 190mm Pan had been the 1st Pan car class established, it might have been more popular, but it's too hard to support 2-classes that so close to each either.