One reason: From half to two-thirds of poll respondents don't actually vote in a primary.
This week back in 2011,during a heated GOP race for the presidency, the leading candidate in the polls was Newt Gingrich. The winner of the Iowa caucus in 2012, and which kicks off the primary season,was Rick Santorum (remember him?). Four years before that, the leading national candidate was Rudy Giuliani and the Iowa caucus was clinched by Mike Huckabee.
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Source: fastcompany.com