There's a number of factors that can contribute to this answer and the debate has raged on for 100 years. Factors such as population density and a dwindling food supply are often mentioned, but I am of the mind that it was the weather. Up until 12,000 years ago the climate around the world fluctuated from hot to cold and back again in intervals as small as a decade in some cases. This makes it extremely difficult to cultivate plants to grow in a certain climate if the climate is never stable. 12,000 years ago marks the Holocene and we have had a relatively stable climate that allowed people to settle down and begin farming. Whether that is, in fact, the case or not is still up for debate but the fact remains that people just were not able to farm before the Holocene.

Richerson, Peter J., Robert Boyd, and Robert L. Bettinger

2001, Was Agriculture Impossible During the Pleistocene But Mandatory During the Holocene? A Climate Change Hypothesis. American Antiquity 66(3):387-411.

/r/askanthropology (or is it /r/askanthropologists ?) would be the place to ask, but I'd search first since they've probably answered it before.

You might also see this question also asked today: Was humanity just wandering around for 10s if not 100s of thousands of years as hunter gatherers before agrarianism?