I suspect the biggest factor is the size of the racer population. More racers mean more races and more super fast guys who drag the pack around. USCF somewhere on the site has a breakout of licensees by geography.

Just fyi, Texans usually compete pretty well when in other states. John Korioth just won 40-44 nats, the juniours/u23's have been placing in their NRC events and the ladies have been placing well in the NRC events.