One of the main reason is "redundancy". Consider two different ecosystems, one with a few species, each species has a different ecosystem function. What do you think would happen if one of those species died (perhaps a disease wipes out an entire population)? Then think of another population that has many species so it has some have a functional overlap, what do you think would happen if you lost one species in this ecosystem? That's true - the more genetic diversity within populations, the more stable it will be too because it has more variations and it is more likely that not all individuals will be affected by a change - like a disease.
I hope that this is the answer that you were looking for and it has helped you.
