The United States is an increasingly polarized country. There are fewer and fewer swing voters, which means there are also fewer and fewer swing states.
That number has declined very steadily ever since, all the way to 14 in 2012. (This is actually a pretty broad definition of a swing state, given only about 10 states were genuinely contested in 2012, but it's the trend line that matters here.)
Here's another way to look at this. Olson calculated the number of consecutive election in which the states voted for one party or another. The darker states vote more consistently for one party or another, while the lighter states have flipped more recently. (Swing states are gray.)
What's striking is that even many of the lighter states aren't competitive -- like, at all -- anymore. Republicans will not compete for California, Illinois, New York or New Jersey in 2016, despite George H.W. Bush carrying them in 1988. And Democrats will not contest Arkansas, Louisiana, Kentucky, Tennessee, and West Virginia in 2016, despite them having gone for Bill Clinton in the 1990s.
Indeed, the days when these states were even on the map for the minority party seems long past.
Some like to think Hillary Clinton could put some red states that her husband won in-play, but unless 2016 is an absolute blowout, that's just not feasible. And it's because our electorate has gotten so much more polarized -- and thus, predictable -- over the least two decades.