In the same setting of San Diego, CA, there are osprey, seagulls,crows,falcons,etc.Why do Falcons not eat fish?

All animals have their own niche which includes prey species they feed upon. Falcons (accipters) are designed to take small animals - especially birds on the wing. If falcons were feeding on fish, they would be competing for the same resources as the osprey. Once you have two animals with overlapping niches (both diurnal hunters hunting for small fish in this example) then you have competition which will end up driving one species out (either out of the area, or out of hunting for that prey species into hunting something else, or even out of hunting at the same time of day). Additionally, most falcons are really too small to take fish of any size that might be grabbed by talons. Even osprey have to face fish into the wind in order to fly as they are almost too small to carry them. The falcon's body makes it much more able to manuever after small birds in flight even in the trees which means there is an available source of food which is not accessible to any other avian predator (hawks, owls, eagles and osprey are too heavy and not manueverable enough to take birds on the wing).