Agree - it was because the early 2.5L cars were the first to depreciate to a point where it was economically feasible to add a limited number of modifications to produce a (relatively) affordable race car with more than enough performance to keep the fun factor high and the racing competition stiff.
There is continuing talk of a Super Spec class for the 3.2 Model S cars but it has yet to get much traction. Also in the works is a Cayman Spec class for the "other" Boxster's with a full factory roof (
).