The first city is not Uruk. Jericho (still inhabited!) and Gobekli Tepe (modern Turkey) were occupied for the first time before 9,000 BC. The transition to a sedentary life in the eastern Mediterranean began around 13,000 BC (Natufian Culture), linked to the management of wild wheat crops and wild ungulates like gazelles (later replaced by goats and sheep) that would evolve over time into agriculture as we know it. The presumed answer is that without the established source of nutrients offered by agriculture, human groups couldn't grow beyond nomadic tribes of hunter-gatherers.

I think a common answer to this is population pressure. Meaning that for a long time (100k's of years) humans did not exist in sufficient populations to form cities.

I am not a human evolutionary scientist (just an archaeologist), but that's my understanding from some of the anthropology texts I've read.

Hopefully you'll get an evolutionary scientist! Although I agree with American_Graffiti that you'd probably get a better answer from an anthropologist. If /r/AskAnthropology doesn't work, /r/Anthropology might, or even /r/Paleontology.

The species is 200,000 years old, but because of a massive impact event in Sumatra (that left lake Toba) ~70,000 years ago, only as few as 2,000 breeding pairs may have survived... which kind of hit the reset button on our development as a species.

This info hasn't been derived from historical sources, but from genetic studies, and anthropological research... so it isn't "history" per se.