That he was retired from filmmaking. (He did go on to make another movie.)
That if he were to make another movie, it would be what he wanted to do. He had earned that right. (He was in a surrealist phase; I have a hard time seeing him doing a film with a Semprún script.)
He had known Lorca, so he couldn't accept any surrogate actor in Lorca's role.
The three reasons are contradictory. 1 would be enough. 2 contradicts one. If 2 is the real reason, 3 is otiose.