http://fivethirtyeight.com/features/why-republican-voters-decided-on-trump/
But none of the folks who are asking why they got the Republican nomination so wrong are asking themselves why they also seem to have gotten the Democratic nomination wrong in terms of everything but the ultimate outcome.
i have really got to write up my demographic snake thing so I can explain to these folks why their assumptions no longer hold. Frankly, it looks more like the last 40 years of politics are an outlier, but they happen to form the basis of most hardcore statistical analysis in political science because we have much better survey data and the dominance of a fairly homogenized demographic group made things look very predictable.