- last post: 01.01.0001 12:00 AM PDT
Isn't it obvious? The Forerunners are gone because they had to activate the Halo system in the past, thus wiping them all out, along with the Flood and anyone else who was around at the time. That's why their technology is still around and being picked up and worshipped by the Covenant. The Halo system destroyed all Flood-possessable life, but left the technology still there.
It's possible that Forerunners = humans. They could have plotted out the course of evolution that created them, and restarted it on some planet (Earth) before they activated the Halo system. Then again, the Forerunners could be any existing race now. Or they might not have even re-seeded themselves (though I doubt that).
Now, about the technology they left behind, I bet you're going to ask why we don't see Forerunner and Flood starships laying around... well, I postulate that the Forerunners really WERE losing the war, and that their fleets were utterly devastated by the Flood... maybe a small handful left, that were mostly destroyed in the last strike to get to one of the Halo installations. The Flood ships, I hypothesize, were biological in origin, and thus qualifying as large-enough biomass, were destroyed by the Halo system's effect.
But, regardless, that's what wiped out the Forerunners. They were losing the war against the Flood, and they seeded a bunch of races around the galaxy before they activated the Halo's, thus destroying themselves and the Flood, and left the galaxy to evolve a new set of races to populate it, possibly including themselves.