It's because an essential protein got misfolded while the body decays, these misfolded proteins can misfold other protein and suddenly your brain is full of useless molecules instead of what it needs.

Some prion diseases, including essentially all those known to affect humans, take decades to cause symptoms. If they hadn't died of something else, they would have eventually died of the disease.

Cannibalism isn't, speaking societally, dangerous because it gives you the disease. If you get it and die, whatever, that doesn't endanger anyone else. The problem is that cannibalism is usually a group affair, and that means one person with a prion disease can pass it on to many people at once, who won't show symptoms for essentially half a lifetime, and they might also be eaten when they die, spreading it to more people, etcetera.