I think it's the coming together of multiple cultures that have immigrated as well as the Indigenous culture.  Bringing livestock over as well as the native animals and using that for food, clothing, etc was a way of life.  Once it becomes a business, then the marketing of it and mass distro integrates into the culture.  Having a bar-b-que on the weekend with friends is very "American".  Livestock is big business and would take many generations to move toward a plant based diet.  I think moderation of both is best for your diet