Not a biologist, so this is just a guess, but I imagine their simplicity is their weakness. Viruses rely on the machinery of nucleic acids to replicate. That m

Not a biologist, so this is just a guess, but I imagine their simplicity is their weakness. Viruses rely on the machinery of nucleic acids to replicate. That machinery is complex and robust and the result of some long period of continuous evolution, which means viruses exists in a fairly smooth fitness space where mutation and recombination are effective means of improving fitness. Prions by contrast seem more like isolated spikes in a much sparser fitness space. Presumably the equivalents of mutation and recombination for prions lead to molecules that almost never retain the capacity to replicate, so they have limited potential to become fitter over time.