There are two dynamics going on here. One is the structure created by USAC to promote racing and create a safe and viable method for holding races. The second is the work of the promoter putting on the races.

USAC created Cat 5 to create a channel for new racers to learn to race and limit their potential damage to other, more experienced racers. But that only works if there are enough racers.

Promoters have to structure events to maximize their gains vs. their costs. Few people are making money putting on races, and if it is financially viable to run separate events for the 4s and the 5s, then promoters will do it.

This is an even bigger issue in women's racing, where women 4s (All women start as 4s) are often placed in women's open events, meaning new racers are going against 3s, 2s, 1s and sometimes pros.

Another thing to consider is that field size varies by region. In some parts of the country, they can fill two Cat 5 fields and two Cat 4 fields in one day. In other parts, they can hardly fill a single 4/5 race.