Only 9% of America Chose Trump and Clinton as the Nominees (nytimes.com)

But they represent a sample of the people who would have voted if they could vote (the first elimination), and who will vote in the actual but not the primary (second elimination).

(I have nothing to say about the people who never vote unless they're not in swing states).

So really it is much closer to 50% of the country that chose Trump or Clinton.

The same is true of Obama and Romney in the last election. It shouldn't be presented as a property unique to those two candidates.