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.
his hoard is far less than 500k..
even in january-february 2009 other people were mining bitcoin. hal finney for instance..
secondly as they were playing around with the first few versions of the code, they were litterally throwing away the private keys and trying different things while they debugged. so the funds satoshi nakamoto could possibly still have access too is smaller than people think. and if he did 'sacrafice' those remaining available funds or is hoarding for 30years.. no one will know unless we see funds suddenly move..