why are dwarf cereal crop prefered over tall cereal crops?

Dwarf cereal crops are preferred over tall cereal crops so that lesser nutrients are consumed for growth and development. A major part of the nutrients are used to increase the yield of grains. Dwarf plants prevent lodging and yield loss. A tall, weak-stemmed crop has a greater tendency to lodge than a semi- dwarf cultivar with a stiffer straw. Under conditions of high moisture and nitrogen fertility, semi-dwarf varieties are less prone to lodging. 

(Note: Lodging is the bending over of the stems near the ground level in grain crops, which makes them very difficult to harvest and can dramatically reduce yield.)