My understanding that one of the issues with USAC which caused the ACA to form in the first place had in part due to Masters racing, and the more flexible ACA breakout, with separating the 35+ and 45+ into 4's and 3's and above.

USAC if I understand correctly was less flexible, forcing 35+ & 45+ new racers to race against 35+ and 45+ guys who had been racing 20+ years, and could hammer it out. The ability to create a 35+4 and 45+4 categories increases the marketability of the ACA races to racers who are coming into the sport later in life. Again this is my understanding, and I can see promoters who find the ability to attract new Masters racers to their events as a reason to race under the ACA banner.

If someone has better information on the reason for the historical break between USAC and ACA, feel free to jump in.