Another explanation would be that we don't have the body and can't conclusively confirm his death. We've been extremely cautious in Iraq, for example, to make certain of DNA results before the military officially pronouces a terrorist dead. They've been ultra-conservative in making such announcements to avoid false-positives. Not announcing Bin Laden's death because the military cannot conclusively prove it at this point would make quite a bit of sense.
Pure speculation: OBL could easily have died from lack of medical care re his kidney failure. He was traveling with a portable dialysis machine still requires power. Given the nature of the mountains along the Afghanistan/Pakistan border region, it is quite possible that he got power cut off to his party, and then had troops surround the area so no medical treatment could get in to him.