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.
From what I know bitcoin was never premined. It was tested and mined from the beginning. 'Pre Mining' is having generated coins right away from the start without mining.
At that time difficulty were like not existent really, so he could mine a lot of coins in short time. But 'premine' is not the word I would use to describe these coins.