Because many predators are perfectly healthy eating non-live food, and it might not be worth the risk. If you got a bunch of cichlids you breed for feeders or guppies or anything, then by all means do it.
I used to have to feed live quite often as my knife would NEVER eat anything else. Now I dont ffed much live. Mostly pellet and frozen krill, misis shrimp, and brine shrimp (for my smaller reef preditors).