Jews attracted to communism were attracted to theoretical communism, in my opinion. They thought it would be fairer and in a communist society Jews could finally fit in and be accepted by the larger society. They believed everyone would be treated the same and the terrible pogroms pumped up against them every Easter would end because the state would not support, promote, or encourage religion. They also believed the power of the state would protect them.
We see that in practice a communist state is even more oppressive and dangerous than an autocratic one.
You can't begin to understand the attraction until you, as a people, have lived for centuries in abject fear.
Paul Johnson does a fairly good job of explaining all this far better than I can in his histories.
I am well-aware that many of the U.S. Jewish Communists of the 1930s-40s-50s were immigrants from Czarist Russia and its history of pogroms. This propelled a lot of Russian Jews to support the Russian revolution (e.g. Trotsky). So, I can understand how some immigrants from Russian in that period might be anti-Czarist. But one can be anti-Czarist without being a Communist (e.g. Kerensky).
It is now 2004...almost a century since those with memories of pogroms immigrated. Their ancestors have become among the most affluent and influential in the entire nation. Capitalism and the freedom of America has made that possible. Yet, descendants of these immigrants still cling to the Left. I just don't get it.