So do American Jews generally, it's not unique to the Hollyweird set. It mainly has to do with the American Jewish population coming from a context of being both highly urbanized and highly centralized in the Liberal bastions of the Northeastern seaboard cities, which until the post-War California explosion was the bastion of Socialist, Communist, and Leftist thought in the US. Of course a lot of their intelligentsia was far left and fled to those locations to escape from the Czar, the Kaiser, Russian landlords in Poland, and various other people in power over Jewish populations who weren't all that keen on revising the existing social order to the overlords' own disadvantage, and that Leftist intelligentsia informed the American Jewish cultural tradition.