@Anonymous #6A1E
Because the greatest existential threat to all life in the Galaxy had infested the Ark, the central control unit for the entire Halo array. They’d been knocked down for a time by Chief blowing up High Charity, but even during the events of the mission leading up to Johnson’s death is was pretty clear it was recovering rapidly.
Keep in mind that not only would the Flood taking control of the Ark be an absolute nightmare scenario in and of itself, as it would remove the option to cleanse the Galaxy again should that prove necessary, as well as handing the Gravemind a platform that it could essentially use to build artificial worlds that it could use as planet-sized infection vectors, there was also the portal above the Ark that was still open. The portal that leads directly to Earth.
It’s just like the sarge said “We don’t have a few more days!”, they had to destroy the Flood right away, while they where still recovering from the last crippling blow dealt to them. The Flood infection can spread very rapidly, a few days would have been too late.
