I can see several reasons for it. First, as a Christian, I see the human person as sacred, and this includes their body. Eating a person is denigrating their humanity when a human being is sacred.
The other reasons I see are practical. Really, how well can you love other people if everyone is a potential steak? Just look at those luscious thighs...
Imagine living in a society where the guy in the alley feels he has the right to eat a human being. They no longer even need to worry about properly disposing of the body; it could've been bought in the market place.
There is the issue of revenge that's already been touched on. You just offed this big man's wife because you were hungry for ribs. You better hope he's a vegetarian in that case. Likewise, imagine eating the recently deceased. People go ballistic over their pets being desecrated, and most people value people more than animals. In either scenario, you would have a short life expectancy if it got out.
The disease issue, also aforementioned, plays heavily into it. Some illnesses just couldn't be cooked out, or worse, may simply linger if cooked improperly. Imagine, again, getting AIDS from your last BBQ sandwich *yech*.
As the final reason, I can't imagine a pizza made from human sweet bread tasting as good as pepperoni
.