I think _why left because he didn't want to do it anymore, it's that simple. Being _why must have taken up a load of time and one day he just wanted to spend it some other way - I'd suggest that it shows he was far more balanced than the average "rockstar" programmer, he may really just have left to spend more time with his family.

He gave a lot to the community, far more in the short time he was around than most people produce in their lifetime, and we should be/are thankful for it.