To some extent I can't believe people were surprised when a disease resulted from them feeding the carcasses of cows to other cows. I know they say that part of the reason the infection happened was that they had lowered the regulation temperature the feed had to be cooked at.
But really, the reason cannibalism never caught on is because it is really bad for you. People in areas that did practice it are some of the only people who suffer from the other kind of prion disease (the laughing disease). We read about it in my anthropology class.
And who knows what other diseases might have infected them that aren't only transmitted by eating brains?
Cannibalism is gross and taboo to us because it is incredibly dangerous. And it is just as dangerous for cows.