Every actor has a vision of a pet project they want to star in, or even maybe produce someday. If you're good, and you work long enough to get clout, that sometimes happens. But Hollywood is fickle, and there are plenty of reasons a movie never gets made. First, you have to get the right people to believe in your project, then you have to get financing, and after that a hundred different things can go wrong, at each stage of the production. Another project gets in the way, people lose interest, and the worst thing might happen.
Forty years later, no movie has been made from the novel A Confederacy of Dunces. Read about 13 film projects that didn't happen, and who might have starred in them, at Cracked.