DC Region kept ITR in their Big Bore for years and completely killed ITR participation.
Why does this have anything to do w/ participation numbers? In some ways it can be easier to be the slower car - you simply drive your car, your race, your line. Let those guys pass you, they bear the brunt of responsibility.

I always assumed that ITR was slow to pick up due to the cost of building these cars is high and the class got started just as we had a huge change in disposable income across the country. All participation numbers fell. Hell, spec miata would not have been so popular had it been launched in 2007.