There are quite a few reasons but I want to preface my answer by saying that Jews differ quite a bit politically here in America. I have plenty of relatives that are all over the map politically. However, there are some central reasons why Jews tend to be more liberal and also lean socialist.
When Jews came to America by in large they were treated poorly, but that was nothing new. There were many diaspora, Jews being kicked out of one place and then another. Part of that has to do with looking different, having different customs and stuff like tat, but a large part has to do with Judaism requiring that no Jew ever pledge fidelity or allegiance to anyone or anything but God. So if you won't bow to the king, then you are on the shit list.
So, The Jews find themselves kicked out and persecuted from one country and then another and then another, always moving from ghetto ghetto (the word ghetto comes from the Yiddish word Shtetl which were the slums the Jews were forced to live in throughout much of Europe.
At the same time that Jews are being persecuted we are a people that come from a matriarchal society that greatly values looking out for one another. The Jews would come to a new ghetto here in America and any Jew that did find a foothold in the new world would be expected to extend a helping hand and lift those around them.
Many of the Jewish holidays and topics of conversation have been about breaking free from bondage, slavery, and getting out from the yoke of those in power. We helped our brothers and sisters come up. But a Jew is not a special creature, our brothers and sisters don't need to be Jewish, they can be anyone else who needs a hand to step up from beneath the pressing devastation of bondage. We were slaves and American Blacks were slaves and we will always show marks of that bondage. We came forth from Egypt 2500 years ago and that lesson still remains. We were under the thumb and the knives of the Nazi's and I suspect those scars will teach lessons amongst the Jewish people for the next 25oo yeas.
So we generally feel kinship with many people around the world and in America who are the downtrodden, who have been enslaved physically, economically or emotionally. Those bonds are strong.
Jews are educators. We have many books but the talmud is a central book that is all about asking questions, not getting answers. How do we learn? How do we grow? What do we do to help ourselves, our neighbors and others who need assistance. So my father marched with Martin Luther King (only once unfortunately) and I wen't to my first protest when I was a kid. We want to grow, as a culture, as individuals and especially as a society. So we feel kinship with many other minorities, Blacks certainly being one. We will learn from them and they may learn from us.
All that was just out of my head, I am sure a lot of other Jews would disagree with me. But that's how I see it.
