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.
Satoshi had much bigger plans for
BTC and he will never touch these coins. it is very likely that he destroyed his private keys in my opinion.

@AGD
these discussions will never end
