But that isn't why, Spec Miata (for example) is the same and has an outsized following. in fact, single-make racing is nearly as common as mixed.
Does Spec Miata have a spectator following? I have always taken it to be an accessible form of motor racing in which to participate, not something to watch.
There was the IROC-Z and I don't know what the viewership was like, but people recognized the car.