What, if anything, would prevent flying fishes evolving to be capable of extended flight?
As AMR said, fish are tied to the water by their respiratory systems. Bats and gliding mammals and herptiles don't have that limitation. Fish have an advantage over mammals and herptiles in that they're generally streamlined. On the other hand, they're adapted to "flying" in a denser environment and might therefore be a little heavy for flight. Also, I wonder if the need to keep their skin moist might present a problem.

Nevertheless, flying fish are halfway there. Just imagine if lungfish - which can survive out of water for long periods - were adapted for gliding.

P.S. I posted this as a comment by mistake, then deleted the comment.