1. They/she/he is waiting for their BTC to be worth FAR more than it currently is before they're willing to touch even a fraction of it. Meaning they anticipate enormous valuation increase within their lifetime.
2. Bitcoin was a gift to the world, and the creator will never touch any of the wealth. The implication being that it was - and always will be - an utterly selfless act.
or
3: it's hard to get fiat out without revealing himself, and at least renewing interest in who he is?