Water, especially during the times of early settlement of the Western states, was vital to trade and civilization. Settlers started at the East and West coasts and moved inland from there. The states you mention, being far inland from the East, and behind the Rockies from the West, were understandably one of the last states to be settled. New York, by comparison, had a decades long head start on say, Fargo. The development of cities is also one of accelerating returns. The factors that made New York bigger in the first place, continue to stimulate its growth at a higher rate than Fargo would have, not to mention the fact that being a large city is a stimulator of growth in itself.