Earlier quoted context omitted.
That possible explanation has been put out repeatedly. Given how successful exit polling was until 2000 in the USA, and given how successful it is in elections around the world, I do not believe that this is a plausible explanation of recent discrepancies. But, as is common in this sort of situation, I have no way to prove it.
Nate Silver wrote a great article in 2010 about why the notion of exit polling itself has a lot of fundamental problems[1]-- among them, the fact that it is extraordinarily difficult to get an truly random sample. [1] http://www.fivethirtyeight.com/2008/11/ten-reasons-why-you-s...
Still the national media had faced those issues and successfully did it for decades. Then their polls stopped working in 2000. Why?