Why? There's a compelling story pointing to Bufalino. Hoffa, whose Teamsters controlled 90% of the country's cargo trucks, was caught up in the middle of the U.S. government, all the way up to President Richard Nixon and the CIA and their work to end Castro's communist rule in Cuba. But Hoffa was also tied in the Mafia and its then growing interest in Las Vegas casinos.
The Mafia, it's believed, thought Hoffa was getting too cozy with the government - their longtime, uncomfortable ally and enemy.