In the book Doublecross, Sam Giancana's brother and Nephew lay out reasons why Giancana helped deliver WV and Chicago votes to Kennedy. According to them, Sam thought that if he did this then the Kennedys would owe him favor.

You do know that it has been proven that Kennedy and Sam Giancana shared the same mistress and she delivered messages back and forth (Judith Exner).

Giancana/Trafficante/Marcello all had the motive and means to kill Kennedy.
I think some rogue agents in the CIA provided intel on where Kennedy was going to be. The US Govt has whitewashed Oswalds and Rubys historys.