"He's Italian."
Well so are almost 20 million other Americans who don't have Pacino's career!
I think the reason why actors often get typecast is a combination of what they are already famous for--the movies and other work they have already done--and the fact that many directors and others think that certain actors have something about them--their physical appearance, something about the way they normally talk and behave, or both--that gives them an affinity for certain types of roles. It should be noted that Robert de Niro is more noted for playing gangster roles than Al Pacino.