I have a theory that it is because so many live in big cities, and like other big city types vote Democrat without thinking. The Jews living out here in the rural West vote Republican for the most part.
So you have several generations of Jewish Americans whose political identity has been based on the Democratic Party of Roosevelt and Truman and who, in their hearts of heart, still see Republicans as the rich, wasp, no-Jews-allowed country club set. Add in the intentionally misleading but universally accepted tags of right-wing and left (National Socialism was VERY much a socialist movement and philosophy) and you have the formula for, as with African Americans, a staunch knee jerk core of millions of Democrat voters.
Finally, many American Jews do not have the visceral loyalty to and regard for Israel that their parents and grand parents had a generation or two ago. Like most good Liberals, they are perfectly ready to see the US and anyone on our side (including if not especially Israel) as the force of evil and oppression in the world. How this self hatred began and been fostered through the education system and mass media of this country is maybe the greatest scandal of our history.