For some reason he can't sell them. We just don't know what those reasons are.
Lots of people can speculate, but the reality is that we don't know.
The ones I've heard mentioned are:
1. He is dead
2. He cannot sell them without risking his anonymity
3. He lost them (machine crash / re-install)
4. He treated the early mined coins as test data and deleted them
5. If Satoshi starts dumping, then the market will definitely crash. If the public believe that Satoshi has lost faith in BTC then everyone is going to want to sell too. It is like putting a gun to the head of his own child and pulling the trigger.
Whether or not he really has 1,000,000 BTC is also debatable I believe. Often people quote this figure as the bulk of the early coins that were mined, and make the assumption that Satoshi was the only one mining. Nobody really knows which addresses Satoshi really owns. It is all "educated" guesswork.
Alternatively, he could slowly sell it off over time, but there's no way anyone could sell that much at once. Of course, he very well could be slowly selling it off right now.
His bitcoins are currently something like gold backing USD at the moment. If he sells his bitcoins now, the entire market will going to fail, because people will loose faith.
He could just sell 100 bitcoins from time to time and still earn a lot of money from it.
But BTC is trading for over $900 now... and plenty of people are paying.
What you do not understand is that Bitcoin is a new technology, that gives us capabilities that we never had before. Just like other powerful new technologies, it will slowly eat its way through every aspect of society until it has changed the world and everyone's life.
The same as happened with electricity, with the computer, with the Internet itself.
If you want to know where Bitcoin is going, take a look at the adoptions of other important technologies:
http://www.uversity.com/blog/wp-content/uploads/2011/07/Comm...