Many natives were farmers, but those farmers were also hunters and gatherers. Hunting provided essential proteins. Europeans evolved to get a lot of proteins

Many natives were farmers, but those farmers were also hunters and gatherers. Hunting provided essential proteins. Europeans evolved to get a lot of proteins from cows milk, but natives are lactose intolerant and so would not have been able to drink milk (http://www.amazon.com/000-Year-Explosion-Civilization-Accelerated/dp/0465002218 http://www.amazon.com/000-Year-Explosion-Civilization-Accele... ). Europeans also evolved to deal with farm animal borne diseases, having natives try and use European farming techniques (with animals) may have been a death sentence. And even if the natives could have adopted farming, very few people will give up their old way of life and ancestral lands voluntarily, without putting up a fight. The low density hunter-gatherer-farmer life the natives lived was in many ways more attractive than the dense settlement agriculturalist way of life.