I think the reason is that he resonates well with a large group of people (hackers) that are smart and able, but are often very introvert. They often don't get the credit they deserve, are bored with the typical business and corporate stuff that they find simple and shallow, and don't understand why they don't get to where they want to be, and why the world is as it is if you are intelligent and introvert. Paul Graham is extremely good at putting these feelings into his essays and giving hackers a hope that they are something special, and that maybe it's not them that are doing it all wrong but the world.
I read a comment here some time ago that stuck: A guy felt disilusioned at the world and typed the word "why" into google (true hacker style I might add) and PG's essay "Why nerds are unpopular" came up in the search results. This essay, and many like it, give meaning to a lot of people that find youtube, myspace, and Fox news to be below their intellect and don't understand why nobody in this world seem to be able to think for themselves and have their own opinions.
That's why.