ok, i have thought a little and have simply added more detail to my theory. I didnt really have to change anything.
so there was a previous galaxy full of other life, until the flood overran everything and wiped it all out. In that time, the flood had evolved from the smaller flood forms into the mighty precursors. They no longer had to rely on hosts and they could build technology at that point. They also had no enemies at the time becuase there was no other life. Then came the belief in the mantle. As being keepers of life, the precursors had no life to protect. So they created the forerunners to put the mantle into action. they wanted to be peacefull with other life because they no longer needed hosts to thrive off of, so they didnt need weapons of any sort. Meanwhile, the forerunners had developed weapons and crated an army to rebel against the precursors. The precursors had been in peacefull times and didnt have an weaponry to use in a war. So they were easily defeated by the forerunners who were prepared for combat. Since they were no longer in a position to keep the mantle, they would have to hand it off to humanity, and not the forerunners because of the war. the original flood forms would have to test humanity to prove that they should keep the mantle, since the Precursors were unable to do so themselves
or it could be possible that this "cycle" of the flood happens over and over again in the same galaxy. The flood destroys all life and becomes precursors...precursors create more life and create the mantle.....the life that they protect rebels and they are devolved to the flood.....the flood test to see if a a new species will be worthy to protect the mantle the next time around....and the flood always seems to win...so they become precursors again, and try creating more life.
and this cycle has continued until humanity fought bravely against flood, and has somewhat defeated it