I'd say that a large part of incumbent advantage is directly related to plurality. I don't know the specific details of the elections that have kept Feinstein propped up. But I'd think the primary would revolve around a bunch of challengers that are each really liked by some people, but really hated by others - so Feinstein ends up getting the defensive vote from people who don't want change towards the popular challengers. Then the general election has the same effect, plus all the people that already "made their choice" in the primary.