[mjbellantoni]

Basically, it's supply and demand.

Satoshi has a large position relative to the whole market. If he every wanted to convert those coins to US dollars or some other currency, I'm guessing the supply it would introduce would cause the conversion rate to crash unless done in a sophisticated way.

[elif]

Satoshi's fortune (1.5Mbtc) is almost as large as the daily volume (1.9Mbtc) of the entire market... its sudden change from dormant and, for all purposes inert, to "potentially market-destroying" would be way too spooky.

Even a single satoshi leaving one of his known wallets could spook enough people to cause a crash. I can't speak for other investors, but even though I have been long in bitcoin for years, I would be out in a heartbeat, for at least a month.