What change in Mesopotamia around 10,000 years ago resulted in a less nomadic lifestyle?

You have to bear in mind that very little is known about that time. We call the people Sumerians, but we don't know if they were one tribe or many. We speak of Mesopotamia as if it were a city, but in fact it was over a thousand cities, something like two million population, and we have not examined those cities because they are in a war zone. The only thing we know so far is that they had no preparations for war or self defense. Any other information is somebody's guess. It is quite possible that what we interpret as "nomadic lifestyle" was nothing more than a group of villagers exploring the countryside.