IIRC it's because when the Flood infest a host, it not only retains their core memories, but adapts them at a rapid pace. The Halo wiki gives a good explanation (I've read it and it's very accurate).
The Flood, a virulent species of parasites, is an anomaly in all known biology, and an anomaly to regular life. This may be explained by its nature as an extragalactic species, originating from a different galaxy.[1] It is extremely adaptive, and mutates hosts through different stages of accelerated "evolution", where the body of biomatter accelerates its own evolution to the point of a massive biological breakdown, where by the infection can first take place. The species can reproduce without host bodies, though this is described as a last ditch effort, and only performed by pure Flood forms.
The only known pathway for Flood reproduction and survival is by the infestation and assimilation of other species. They do this through a form of cellular reproduction known as lysogenic replication where the viral cells infect host cells with their own genetic code which mutates the host cells into viral forms.
Capable of surviving extreme environments ranging from -75 to +53 degrees Celsius and even underwater (in early stages of life), the Flood can withstand harsh environments of all known colonized planets.
It would seem that the Flood can infest any living organism, but that they prefer hosts with a large brain capacity to accelerate the formation and expansion of the Gravemind hive intelligence and the Flood intelligence as a whole. They are only as intelligent as the beings they infect; the more cunning the enemy, the more dangerous the Flood will be.
As the Flood spread and replicate, they will alter the environment to better suit their needs. This takes the form of biomass growing on any available surface, and the air being filled with spores. This appears to be part of the Flood life cycle and is done automatically, and leads to the development of a Gravemind and a Flood Hive.
The Flood seem to thrive in moist and humid areas, which lends itself to the fact that the Quarantine Zone is bereft of organic life, and is cold and perpetually snowy, to hinder the development of the Flood.
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