Because it was never activated. The Index (the activation key) wasn't placed in the control center until the level Two Betrayals, their last day on the Ring, and then Cortana stopped it by erasing the Index's activation protocols and downloading them into her own subroutines, so only she could activate the Ring. So the Ring never had the opportunity to begin charging in the first place.

I see thanx sry its been a while (6 months since i re watched the story an few years since i played it) thanx for the intel posting

No problem. Another example of the Ring activation time is in halo 2. The final boss fight with Tartarus takes place entirely while the Ring is charging, and in the window of time, 343 Guilty Spark will eventually say that the Ring is about to fire. So it takes about 15-20 minutes.