Why natural selection might favor traits that keep 2 individuals from populations that had been reproductively isolated for a long time from mating (pre-zygotic isolation), even if hybrid offspring technically would still be possible if mating were to take place?

Pre-zygotic isolation prevents a zygote from being formed that cannot survive or provide more offspring in the future

Explanation:

If pre-zygotic isolation was not in place hybrid offspring would be possible, but then there might be hybrid inviability, sterility and breakdown (post-zygotic isolation). Inviability when the hybrid does not survive to reproduction. Sterility is when the hybrid is not able to produce offspring. Breakdown is when the hybrid is viable and fertile but its offspring is not. Pre-zygotic isolation is in place to prevent wasting energy on reproducing and creating offspring that would not be able to add to the species.