Maybe because they finished the fight with the flood and the covenant? And before you say "but there is potential for other fights to happen" it doesn't matter because it wasn't talking about every fight ever that could ever possibly happen.
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Except the fight with the Flood wasn't finished.
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With the flood from delta halo it was.
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There would still be other flood out there. In Halo 2 it was shown that there can be more than one Gravemind since the one in Halo CE is the base for one and it's younger than the Halo 2 Gravemind.