It takes years to decades before kuru disease exhibits onset. I highly doubt people could even connect the disease to cannabilism let alone modify their behavior.
It is genetic, we have a genetic predisposition to not want to eat our own kind (like most animals).
The reason is also probably partly non-genetic in the sense that evolved due to real harms like kuru. But more likely conentional disease is a far more likely reason. Virus and bacteria you catch would be particularly good at attacking humans and would cause all sorts of issues.