Everyone, he did not leave because of the making of Heart Gold and Soul Silver. He left about one month before the games were even announced. None of us know the real reason why he left. Maybe he got a full time job, maybe he got married, maybe he decided to quit hacking. Maybe we will never know, however all we do know is that there is a very good chance he will not return. However what makes me very curious is why he never even told us he was leaving.