Why do some Person get more famous after they're dead?

They do all their best stuff when they're younger and then move into obscurity as they get older and their audience gets older or moves on to other stuff. Then once they've died everyone "rediscovers" their stuff amongst a wave of nostalgia and they become even more famous.

Wait until Paul McCartney dies... I'm not looking forward to it as (fantastic though it is) Beatles music will be everywhere and then there will be the flood of movies based on their lives and then the books and then all the teens will "discover" The Beatles and all hell will break loose.