Their hand was forced (After the Primordial's supposed "death " Precursor artifacts came into play) and once the truth was shown about the Precursors and the Flood, and what the Forerunner did, they decided they were no longer worthy of the Mantle.

Why did the Forerunners die?

Downfall. Though thriving for millions of years and achieving extraordinary technological feats, the Forerunners eventually fell prey to the horrific parasite known as the Flood. Their efforts to contain the parasite culminated with the activation of the Halo Array, eliminating all sentient species in the galaxy.

Why did the Forerunners not save themselves?

The Forerunners had a plan in place to preserve themselves. They would all go into stasis in their shield worlds and emerge after the rings had fired. The problem is the AI Mendicant Bias they had created to control their defense against the Flood went rampant and betrayed them.