Why did american farmers prefer to settle in the northwest rather then the great plains?

Better land and weather--The land of the Great Plains is dry and has a thin layer of top soil. 

The Great Plains has undesirable land for farming. It requires additional nutrients and often irrigation to make up for the lack of top soil. Also the Great Plains has the threat of dust storms during drought, tornadoes, and a winter season where growing cannot occur. The Northwest has better land for growing plants and a more favorable weather pattern.