The US does not even have National Championships for pool except for Juniors. We have national snooker championships and national three cushion championships, but no national pool championships. It has to do with our pool governing body, the BCA.
The closest we have come to a pool championship for seniors was with several senior pool tours, which I believe included one Steve Mizerak was associated with. There are occasional seniors-only tournaments but they are open and not national championships.