I have never completely understood why, but it is what it is. I can only suspect that it is because so many Jews live on the Left coasts.
I think it has more to do with the concept of secular reform Judaism and tikkun olam, the pogram fear meets collectivist anti-pluralist bent of Ashkenazim immigrants from Eastern Europe from 1890-1935 and a natural distaste for Christian traditions which influence social behavior...and the concentration in large urban rather than pastoral areas.
I know that sounds like a lot but I think it explains why American Jews today behave as they do...considering how bright many are.
like another FReeper here always says....Moses to Marx in 12 easy steps...it ain’t random....I applaud the minority of Jews who buck this trend...they are under a lot of pressure to conform to the collectivist bent of the tribe