I think he just took sufficient steps to remain anonymous. Probably not that difficult to do if it's something you're conscious of before the get-go. Most of us here after all are mostly anonymous but with a few additional steps most people could leave little trace of things that could be tied back to them.
I'm more curious why satoshi was hiding his identity from the start: did he know how big Bitcoin would get?
I don't think it's common for programmers to take measures to protect their privacy when they post a beta on SourceForge.
Nobody could have predicted where bitcoin would go but I believe he did say something along the lines of (and I'm colossally paraphrasing here): In ten years time either nobody will be using it or everybody will be. There are a lots of potential reasons why he may have wished to be anonymous, but we'll probably never know his true reasons. Maybe he wasn't sure of the legality of it or didn't want to run into any potential legal problems in the future. I'm sure governments would have wanted to talk to him at least. If someone knew who he was then they could certainly pressure him to take bitcoin in a certain way or even try to sabotage it. Maybe he just didn't want the attention or hassle of being the founder/figurehead of it. Being famous in any sort of capacity for me would be a nightmare. I like my anonymity. I like being able to walk down the street and nobody hassle or bother me for anything. That changes when you're famous. Maybe satoshi also felt this way.