This is one of the big questions of the 2016 election. It is also one of the hidden problems that needs to be solved if American democracy is going to thrive again. I found this video article that helps explain how it happened. It makes good sense.
http://www.bbc.com/news/video_and_audio/headlines/37802358
The next question is, “What can we do about it.”
1. More Americans need to vote in primaries. Primaries are very influential. It is intellectually tempting to be an “independent.” But the number of people who are uninvolved in primaries is part of the problem. I was an independent myself for several years until I realized the power of the primaries. Then I registered for a major party so I could express myself in the primaries. I can still vote for whomever I choose in the general election.
2. Primary voters need to keep “elect-ability” in mind when casting their ballots. Primary voters who vote for extreme, irascible or unqualified candidates who will be greatly handicapped in the general election are asking for their candidate to lose in the general election. Primary voters must strike a balance between where they stand and how electable the candidate is. And the farther to the left or right a voter is the more they need to think this way. To fail to do so is to endanger the chances of the party of your primary in the general election as is happening to both parties this year. This year it is obvious that either party would have had a cakewalk with a moderate candidate. If two moderate candidates had been put forward, we could have had a real democratic election.
3. This year’s Republican process is making a case for some kind of “vetting” by major parties in order to run in their primaries. A major party should not be put in a position as the Republicans were this year where a person they cannot truly support squeaks through with popular vote. But there is danger with this idea too as it opens the way for power player control and cronyism in the vetting process.