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 only wanted to design it and he didn't expect earn huge amount of money from bitcoin, so he was unhappy to see so many people are speculating bitcoin, so that's why he never sold any of his bitcoin.
The other reason may be satoshi is dead, or CIA/FBI wants him, so he has to keep anonymous, there are so many scam cases and money laundry cases of bitcoin, I am pretty sure FBI will arrest him if he is not anon.