Why have the northern plains states voted consistently Republican in Presidential elections since 1940?

Beginning in 1940 (with Wendell Wilkie), the four northern plains states of North Dakota, South Dakota, Kansas, and Nebraska have voted Republican in all but one Presidential election (the Johnson landslide of 1964). That is a more consistent record than just about any other group of four U.S. states.

Why was that?

(In answer to some comments, 1940 was long before the Republicans became the "conservative" party in the 1970s and 1980s, and only four years after FDR swept all four states in 1936, including the home state of his opponent, Alf Landon (Kansas). I would expect the answer to be along the lines of party differences in agricultural policies (tariffs, price supports), over long periods of time, but I don’t know enough about these matters to answer the question myself. The answer might even hinge on the difference in policies vis-a-vis e.g. wheat vs. corn, because nearby Iowa doesn’t exhibit this consistency.)