The answer is — most were raised as “Red Diaper Doper Babies” — with DNA from Eastern Europe’s Communist Leaning Jews...
They felt more allegiance to Stalin and the Communist forces that “liberated” them from the Nazis and then enslaved them.
Many remain Communists to this day — but under new more “acceptable” labels: Democrat, Liberal, Progressive, Leftist, etc...
Forget the "doper" part for a second, which didn't come into the picture until the ultraleft got into the "social revolution" of the 60s. That certainly wasn't predominantly Jewish, although Jewish youth participated in disproportional percentages.
There were a few "Red Diaper Babies" in just about every Jewish community in the late 40s and 50s, although they were a distinct minority within a minority.. The large majority of American Jews during that period were distinctly patriotic and anti-Soviet, and many fought the Reds successfully for control of Jewish and predominantly Jewish institutions, e.g., unions.
So you are overestimating the percentage of (nominally) Jewish Commies during the post-WWII era.