I think Tom could not forgive Paramount for not offering him enough money to appear in III.
Yeah LOL... I posted here a while back that I saw an interview with Duvall and he said something to the effect "Al is the bigger star... but atleast offer me half of what you are giving him"... I guess they really lowballed him to get more money for the vineyard.
I am suprised that no one really see's more possible conflict that Tom may have had towards Michael between 2 and 3. Yet another storyline...