That they don't spread through the same mechanisms that viruses and bacteria use. You'd have to have much more direct contact between the organisms to transfer prions from the one to the other, they are internal and don't self-replicate in the way that bacteria and viruses do. The lack of selection pressure on prions is what keeps them in their lane.

But: consuming bush meat, brains, feeding dead animals to other animals, being careless with sharps and so on are all excellent ways to contract them.