>>535217454
There's a billions year old sentient bacteria colony that calls itself Sublime that's possessed various mammalian hosts over the eons. Humans ended up being it's go to host ever since we showed up on the scene and it's kind of hung out in the background of mankind's history. Come the modern era, it discovered these new fangled mutants that keep popping up are completely immune to being possessed by it for some reason, and it also deduced that, eventually, mutants WILL become the dominant species as humans are slowly but surely out-bred. So, it started a decades long PR campaign to rally people against mutants as a concept so as to ensure its own continued existence by possessing a continuing stream of human bodies.

That is quite literally the cannon explanation. Also >>535218452