Because they are the only entrenched "immigrant" group which refuses to stop voting like immigrants.
For example, when the Irish came to the USA to flee the Potato famine of the 1840's, the Irish voted democratic and for over 100 years, they voted democratic. Once they stopped pining for Ireland, the Irish started to assimilate into their new home country. That assimilation also helped made their voting patterns more diverse.
Even though in places like Boston, they talk alot about the "Irish Catholic" vote, in a country with over 40 million people of Irish descent, I don't that they consitute the monolithic voting block they once did, even as recently as 40 year ago.
However, American Jews have never taken off those "immigrant" clothes, even those who can trace their American ancestory almost to the Mayflower. They can go to Israel at any time, they have somewhere to go if America get's bad. Where do Americans, who don't have automatic Israeli citizenship go?
The US Jewish/Pro-Kerry voting block is very disappointing. I am starting to conclude that those pro-Kerry Jews don't care about either the country of their birth (USA) or the country of their people (Israel). They don't care about themselves. I think they might be suicidal lemmings taking the rest of us down with them.
American Jews were prevented in many ways from joining American society until quite recently. Now that that has changed, I have high hopes that American Jews will take off the "immigrant clothes" and find their way out of the "liberal" corral.