What turned Hollywood into a leftist monolith were the passing of the entrepreneurial founders of the industry and its acquisition by large, bureaucratic corporations. These new owners tended to be run by administrators who came mostly from elite East Coast universities and shared a leftist, secular viewpoint. Michael Eisner, former Disney chief, is a prototype of this type of administrator. Starting with the rise of television around 1950 and continuing to the present, the movie audience became increasingly fragmented. Thus, you could alienate a portion of your audience and still make money. A conservative and Christian boycott of the current industry would be pointless; they largely do not attend movies or watch cable channels like HBO anyway.
And in no small part assisted by Soviet agents of influence. That's what's so laughable about how they're trying to pin Trump's election on the Russians.