People in artistic professions are generally (though by no means exclusively) drawn to liberalism, and Hollywood is no exception. The origin of this is too subjective to give a determined answer. A liberal might argue that someone in an artistic profession is prone to being more open minded; a conservative might quip that getting 25 million paychecks for relatively little work is the ultimate welfare scam. Once established, however, there is undoubtedly a degree of positive feedback: a majority of liberals in Hollywood causes others, who may be apolitical, to gain liberal sympathies and so on. And of course, it is no surprise that liberal beliefs lead to supporting Democrats.
Another rather controversial answer is that at some point, Hollywood became so firmly liberal that conservatives are blackballed. For example, conservative pundit Ben Shapiro’s book Primetime Propaganda: The True Hollywood Story of How the Left Took Over Your TV includes several interviews where prominent producers, directors and actors, unaware of Shapiro’s politics, expressed distaste with conservatism. Others took it farther: when asked if conservatives were discriminated against, Nicholas Meyer (director of The Day After and two Star Trek films) replied “Well, I hope so.”