In the film "Godfather Part III" what excuse is given for the absence of Robert Duvall as Tom Hagen?

In the films, "The Godfather" and "The Godfather Part II" Tom Hagen is portrayed by Robert Duvall. In "The Godfather Part III," set in 1979-1980, Hagen is said to have died some years before in an unspecified manner. So his role in the story is greatly missed. According to the director, Francis Ford Coppola, with Hagen gone an essential character and counterpart for Michael Corleone (Al Pacino) in the movie is keenly missing.

When the "The Godfather Part III" was being made, the initial draft for this film had the character of Tom Hagen in it. Robert Duvall wanted $5 million to reprise this role. Paramount Studios turned him down. The part was recast and altered for George Hamilton to play a new character who was similar to Hagen. The replacement character played by George Hamilton was a lawyer, B.J. Harrison. A line of dialogue was inserted in the film that explained Hagen had died years before.

For what it does in the movie, some film critics have stated that George Hamilton’s clone character of Tom Hagen is actually quite good. BJ the lawyer has some interesting things to say and do. He attempts to recruit Tom Hagen’s son (who is a priest) as a spy for the Corleone crime family before Michael Corleone calls him off.