Diamond says: "An observer transported back in time to 11000 bc could not have predicted on which continent human societies would develops most quickly, but could have made a strong case for any of the continents." Why does diamond begin his story at this point in human history why not sooner or later?

Diamond uses this date because it predates the so-called "Agricultural Revolution" that he credits with the development of global inequalities. This event led to the formation of stratified, complex societies. These fostered the so-called "guns, germs, and steel" that enabled Europeans to exert their influence around the world. His point with the quote in the question is that nothing about the peoples that lived in these regions at the mentioned time would have enabled an observer to predict whether they would have developed agriculture. This, in fact, is central to Diamond's thesis. It was not, he argues, anything inherent about Eurasians, but rather certain unique aspects of their environment that fostered the development of agriculture. In other words, Europe's global dominance, and the development of advanced technologies, was an accident of geography more than anything else. If he had begun his story after this period, we would have already been able to see these developments taking place.