Some reasons for remaining hidden are obviously known. His life (or their life, if it's a group of people as some speculate) would be in danger.
Some are saying, none of the original coins have moved. Yes, the first million coins have not moved ever except for a few transactions. But do we know if Satoshi (the person or the group) actually stopped completely mining? Might he or they not have tried mining with GPUs when everyone else was starting to? That's another few thousand coins worth.
If he didn't need the money before, he certainly doesn't need it now, but at current prices anything saved up AFTER the first million mined ... however unlikely, it's possible he or they were able to continue either mining or even trading. Many others have. Those later coins and transactions would be completely under the radar.
Yes, it is likely that his (or their) life would be in danger. Speculation around the topic probably doesn't help that, but as the founder of the crypto asset industry, there will always be rumors about Satoshi's true identity (or identities). It's very likely that Satoshi is still somewhat involved, and maintains a vested interest in Bitcoin.