- He is afraid of beeing found when he spends one satoshi
- He forgot his wallet passwords
- The harddisk with the wallets crashed
- He is dead
(Most likely in that order)
Collectively "Satoshi Nakamoto" is waiting for mass acceptance before revealing himself and starts dumping coin in market.
If there is mass acceptance, dumps won't be necessary. He won't need fiat.
The time to dump is now, while he is a multi-millionaire and bitcoin's future remains uncertain.