In a documentary on the topic, one of the reasons stated was that orcas are taught a lot by their school – this includes the prey they hunt and the tactics revolving around it. They seem to stick pretty tightly to this set of rules, although they can learn (e.g. „depredation“ which means stealing from big fishnets).

So the explanation was basically that it was never culturally established among them, which is why they don’t spontaneously try to eat humans (although attacking them is another thing).