Why wasn't Clemenza in Godfather 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...