JMO, but I think it is familiarity. Sort of like name brand clothing or food.
I tend to agree with you. It also has to do with looks. Kennedy was good looking and Nixon was not particularly and he was old. Clinton was good looking and kinda funny and Bush I was OLD and boring. Bush II was good looking and Gore nor Kerry are not and they are boring. I think it had a lot more to do with the person running against these Dynasties then the actual families actually becoming dynasties.
Looks and personality are certainly important. I’ve read studies have shown that good looking, tall men tend to be more successful.