Perhaps he can't take it because of repercussions in his physical life - perhaps he works for a central bank!
Or perhaps he's super humble and prefers to let the work speak for itself.
There are many "rich people" around the world but their wealth isn't usually found in an untraceable, liquid form
If someone steals Satoshi's bitcoins and leaves him in a ditch (so that he can't tell people they were stolen), as soon as that somebody starts spending them other people are going to start looking for him (mistaking the thief for Satoshi, at least until they connect an identity to the coins.)
However this does not afford you any protection from robbers that believe, mistakenly or otherwise, that they can clean the coins in a mixer.
- He's just too humble to want the fame. Gavin Andresen could then be a likely candidate.
- Scared, maybe by what happened to Julian Assange. However, unlike WikiLeaks, it's hard to imagine what law Bitcoin breaks.
- He wanted to make money off it and move on. That would suggest Jed McCaleb, who founded and then sold MtGox.
Other reasons?
A determined government could probably concoct something if it became determined to do so.