Because if he had used his huge stash of bitcoin, it would look like a huge premine (around 2 and 3% mined by him, the dev)
Also, the block rewards are supposed to distribute the bitcoin, and not centralize all the bitcoin mined in 1 person hands.
Therefore, i think Satoshi prefered to sacrifice those premined coins for the sake of a better distribution of coins.
So if Satoshi comes to me and wants to unload a 100,000 coin account and I give him 200usd (pick a number) a coin.
That is 20,000,000.
I put it in an unmarked swiss account.
I hold the coins until 2020 then cash them out.
So in 2020 everyone will think I am Satoshi.
Since in reality I am Bill Gates, a Saudi Prince , Donald Trump , Queen of England. Anyone you choose. I was just looking for some skin in the game.
My point is the so called first 500,000 to 1,000,000 coins that are supposed to be Satoshi may all belong to anyone above. Satoshi could have fully offloaded all the coins last year or two years ago. To anyone looking to get into the game. Terms of the agreement include a 5 year no cash out of coins rule.