This is fairly typical. In 2014's mid-term elections, voter turnout was so low that it hadn't been that bad in 72 years.
I think there are a lot of reasons for this, but there are three that come to the top of my mind.
1) Democratic Apathy: It's always struck me that the Democrats generally have a consistently low voter turnout. I think part of this has a lot to do with the fact that Democrats just can't be bothered. It seems that they have a high instance of, "I don't need to vote, someone else will" laziness that constantly lets the opposing party win.
2) Republican Voter Suppression: In 2010/2011, hundreds of bills were introduced into state legislatures that were designed to restrict voting. In 18 states, all led by Republicans, these bills became laws, These laws were targeted to do as much damage to the minority vote as possible. The Brennan Center for Justice at New York University Law School found that following the election of President Obama in 2008, 7 Republican-led states passed laws making it harder to vote. Nine of the states with the highest Hispanic voting populations have also had restrictive laws enacted by Republican-led legislatures. When it comes to voter registration drives, Blacks and Hispanics register through drives at twice the rate of white citizens. The result? Laws in multiple states that restrict registration drives. This is certainly not a comprehensive list of the restrictions enacted to keep minority votes low. So as much as I hate the "race card," it's clearly being played, but it's being played against minorities by Republicans.
3) Voter Disillusionment: Voters of all political views have become disillusioned with the government and its elected officials. We have a legislature paralyzed by Republican obstructionism, an administration fighting back against obstructionist legislators, his own failed policies, and a race to establish a legacy, and a judiciary that instead of ruling objectively often engages in judicial activism. We also have elected officials who are in the pockets of giant corporations and a country that has not, for a very, very long time been a representative democracy (we've been a de facto Oligarchy for decades, now). So voters don't turn out because they don't believe their vote counts. They don't think it will matter.
Maybe this election will change things, but I doubt it.
The fringe Republicans will get Trump in office because the Democrats won't be bothered to vote. That's my prediction.