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.
He probably mined and sold plenty just not the very early coins that people have identified as his. So I think he is doing fine financially. I don't think he "sacrificed" the original stash or lost the keys either. If he tried to sell any of them (other than all in one big dump) he'd crash the price anyway, devaluing the rest of his coins.