I don’t know that the reasons are so clear.
In 1920 the Socialist Debs won 38% of the Jewish vote and the democrats won 19%. In 1928 the democrats won 72%, in 1932, Roosevelt won 82%, then 85%, 90%, and 90% again as president for life, Kennedy got 82%, Johnson 90%.
Clinton 80%, then 78%, Gore 79%, Kerry 76%, Obama 78%
It could be that they were just voting liberal.
Who cares what FDR or Hitler did 70 years ago. He had a bunch of Jews working for him in his command yet he was killing Jews too. Obama has a bunch of Jews working for Him too. What difference does it make to the conservative Jews on FR?