The P1/2 is a NRC race and subject to bike measurements. The other catergories (excluding women) are not.

There was a proposal several years ago that actually made it into the rulebook that all bikes would need to be UCI compliant in a couple of years. That got killed off and we are left with the rules we have now.

The philosophy is that professionals and those who compete at the UCI level or are trying to qualify to compete at UCI events (ie. Worlds) should have to use the equipment they would use in those races. National Championships also got tied into that formula. Otherwise it is "run what you brung".