Because the movie was finished when he was alive but was released 4 years later. Haven't you ever noticed when an actor-actress passes away one year that a movie release with them in it might hit theatres/television the next year? That's why in most cases when a movie is being made, they film the last part first just in case the main star dies during filming. That way they can rewrite the script in the middle and use special trick photography or doubles/look alikes some what. That way they have it all covered!