There are probably a myriad of unknown political and personal reasons.
However, IMO, a deep-rooted cause that no one talks about is that the Kennedys innately consider themselves American Royalty.
The Kennedys consider the Clintons middle-America hicks.
The Clintons despise the Kennedys for being wealthy and powerful enough to roadblock their own agenda for power.
Never the twain shall meet between the two families or the followers of each.
Although the feud has a hint of the Hatfields and McCoys, it's basically a class warfare thing.....the elite vs. the trailer trash.
Leni
It's a power struggle for control of the party.