I can only speculate as I wasn’t around then. But, based on people I know who met him when he would campaign in Mass, he was a rich, good looking, charismatic guy with a gorgeous wife. After 8 years of Ike the US was ready to go younger.
Then he got shot. In our living room.
It had nothing to do with accomplishments, which we can all agree on as being mediocre.
He and his brother Robert were screwing anything female on two legs.
I think you pretty much nailed it there.
Why FDR?
I never did think Jackie was anything special either,