68 Uhm... I'm not certain you read my previous post correctly.
Corn, beans, and squash each contain different subsets of the amino acids that humans need, but cannot produce on their own. They are easier to farm (I assume) than it is to harvest whatever wild game there is in Arizona, New Mexico, and the rest of that region, so when they were brought up from farther south, the tribes began cultivating them. None of them individually produce all of those amino acids, and neither do any two, so before all three were available, they had to get the remaining essential amino acids form other sources. But once all three were available, they didn't need to rely on fish and game much.
Grains also have the advantage that they can be dried and stored for long periods of time, which is difficult to do with meat.