Why didn't Marlon Brando play Vito Corleone in Godfather II?

Answer 1

Brando was never set to reprise his role as Don Vito Corleone in the last seen where Michael announces he's joining the war effort, but during the first movie Brando's shabby treatment by the studio made sure he didn't come back for a cameo

Answer 2

Although the character Clemenza was played by Bruno Kirby the actor, Richard S Castaellano (the original Clemenza) was set to play the role that was later changed to Frank Pentagalie, but Castaellano and Coppola couldn't reach an agreement over the script. Casteallano would only return if his girl friend could re-write his part to which Coppola refused or in movie view... he apparently died between first and second film...