Look at some of the numbers and try to figure out why some crops need irrigating and some don't. Rice actually grows underwater in flooded fields. It's no wonder that all riceland is irrigated. Other crops are grown in the South where there is normally ample rainfall during the growing season. Other crops, such as corn, don't rely on irrigation. Maybe it is because it is grown in huge amounts in the Midwest, where it would cost too much to pump the needed ground water.
Source: Day, J.C., and Horner, G.L. 1987, U.S. Irrigation,
Extent and Economic Importance,
U.S. Dept. of Agriculture Information
Bulletin 532