Fidel Castro saw many virtues in both men also.
It was pure Cold War politics. Janet Jagan was a Marxist Leninist so Castro became interested. The USA, fearing that Guyana might become another Cuba got the UK to rein him in and concocted a CIA plan to remove him. Castro and Moscow concocted their own plan to keep Jagan there.
Roll forward to the 70s. Moscow determined that all left wing parties should be supported, not just the Marxist ones so embraced people like Burnham, Manley and Bishop, and even lent a friendly ear to Eric Williams.
Jagan was told to cooperate and like the little commie lap dog that he was followed their orders to befriend Burnham even though he knew that he was a monster. Fidel was in almost as bad shape as by then he had done so much damage to Cuba that it couldn't survive without the USSR, so had to do as it was told.