Fame? Recognition? To say how happy/unhappy he is with the direction bitcoin is going? What?
Those are all understandable reasons -- he/they is human after all. I think the most likely reason he would come out of hiding would be if he plans to sell some of his old coins. Those addresses are monitored, and any movement would likely cause a market panic/crash. Also, it might be tough to stay anonymous when trying to withdraw after selling on an exchange. So Satoshi would have an incentive in that case to come forward, and explain why, when, and how much of his stash he was planning to sell.