By the time most of the field were in Lola 330/332s, some of the appeal had gone, but also the closeness of the racing. Arguably a similar thing happened with F.Atlantic/Pacific - when it was Ralt RT1s, Chevrons B34/39s and March 76B/77Bs - racing was much closer than when everyone got RT4s - wasn't it?

Both F5000 and F.Atlantic started with multiple constructors - and both prospered. In time, both became dominated by one-make, or a small handful at the most, and the categories petered out - coincidence?

There is possibly something much simpler. How much horsepower did a decent SBC spit out in 1969/70 - 430/450? By 1973-75 they had in excess of 520 - and the fields spread out more and more. A bit like Can-Am - the faster they got because of more power, the quicker the spectacle diminished.