Possibly because of the available flora and fauna. The fertile crescent had foodstuffs like chickpeas which they could domesticate and which were calorific enough to be worthwhile settling down from hunter/gatherer lifestyles and becoming settled farmers. In the Mississippi area the available wild plants may not have been calorific enough to offer a very secure prospect for anyone trying to do that. I think it took ages for corn to travel north from the South American civilisations, and then I believe the American continents didn't have a lot of opportunity to domesticate the local animals. I think they just domesticated dogs, whereas they had stuff like goats much earlier in other parts of the world.

There's a great book by Jared Diamond that attempts to explain why different parts of the world moved faster than others, it's called 'Guns, Germs, and Steel'.