Several old ABA teams transitioned into the NBA and were successful. They still have their team decades later. Louisville would have been one of them.. Plus it would have been good for the economy which means growth to a city. About that time I would guess Nashville was about the same size or smaller than Louisville. Look at the difference in growth and wealth of the two cites. Now Nashville is major league. Louisville not so much.
It was a terrible decision not to take that risk and go major league pro