Hollywood backers are afraid of vocalizing their support for Sanders because his policies and ideas are incorrect. They do not want to be associated with someone like him. This is true for common people as well. The ideas expressed by Sanders do not reflect those of the American public, thus people are afraid to endorse him.
Despite his progressive values, many an Hollywood who support Sanders are remaining quiet to avoid being out as against women. Sanders represents what many in Hollywood want in a candidate and could be a strong contender against the likely Republican nominee Trump. But many are choosing to wait until the Democrat nominee is decided to voice any strong support.
I am sure that some in Hollywood would support the efforts of Mr. Sanders, but I sincerely doubt that many of them would have the nerve to stand and up and vocalize support for any candidate. The cost of potentially alienating paying movie-goers would far outweigh any conviction they felt.
Many in Hollywood have come out in support of Sanders. Seth Macfarlane has campaigned for Bernie and so has Ben Folds. I think that many political pundits have been afraid to show support for Sanders because if he loses the nomination they will lose opportunities in the Clinton administration, but Hollywood backers has no need to hide their support.