Because people want to know who Satoshi is, and the fact that his identity is unknown increases the mystery and the curiosity that people have.
Add to that the fact that he owns 1 millions bitcoins. He can crash the price instantly.
In the wired article i read that he will not sell before 2020 but after that we will see maybe some very dark times. If 5% of all bitcoins enter the market (he also pledged some of his coins to other parties) it will be quite some blood on the streets.