The Precursors weren't going to give the Mantle to Humanity, because Humans and Forerunner were pretty much the same thing back then: A race of hominids that the Precursors had uplifted a few million years prior. The Super-hominids had deviated from the Precursors' intent, as creation is wont to do, and the Precursors decided to gift the Mantle to a faction (Pre-Humans) that was more in line with the plan. Everything else pretty much got written out of the will.

Except, the other factions of the Super-hominids went and did that whole deicide thing. Which pretty much meant that the Pre-Humans and Pre-Forerunner weren't on speaking terms anymore. In fact, it meant ten thousand years of civil war.

In the present day, Humans and Forerunner are separated by seven million years of divergent evolution and bitter history. The reasons for the have been lost to the ages as the story of the Precursors passed in and out of myth, and ultimately became less important than recent events like the Atrocity of the Azidian Expanse. The Forerunner have pushed it out of memory, out of guilt and a refusal to believe that the Mantle is not theirs. Some historians in Human Space still retain records of the event, but few pay attention to them.