[rcxdude]

It's because the prion protein is in the brain. As in, there is basically only one protein in mammals which actually has this problem (that we know of), it's not a general issue with proteins. The variants which exist are essentially because of the small variations in this protein across species, or specific inheritable mutations which make sponteneous formation of the misfolded form more likely.

(this protein, by the way, is not essential for life, but lacking it seems to slow brain development in mice)