I suspect the issue is that, prions require a fairly large series of coincidences to be viable, so the biological niche is a pretty small one. I think they're

I suspect the issue is that, prions require a fairly large series of coincidences to be viable, so the biological niche is a pretty small one. I think they're much more specific conditions than that required of viruses. In addition, your body probably has evolved to use proteins that aren't particularly susceptible to misfolding AND accumulation. (Perhaps a long time ago, it WAS quite common?)