There is a cycle to on-road RC, and F1 is no exception. Most of the on-road classes and racing slows down during the summer. It seems people want to go outside, vacation, spend time with familes, silly stuff like that. Also, it is the off-road season, when all the outdoor dirt tracks are hopping.
VTA is still strong, USGT is building up, and just recently 12th scale has regained some followers (we had a 10 c ar main 2 weeks ago).
As someone not participating in it (F1/USGT) but looking in, it seems you have a lot of the same people doing USGT and F1. The motor is the same, the times are not horribly different, and this is a way to get those F1 chassis's being used in a faster more advanced class. VTA is my slower class, 12th scale my faster. This is for those that can do USGT as their slower and Prototypes as their faster, and as they already have the chassis and esc, it's a motor, tire, and body swap and good to go.