Actually, the article says that only nine percent of the country voted for either Hillary or Trump. While this sounds alarming, I think it's a little misleading, not least because around a quarter of the population is under 18 and thus ineligible to vote.

My understanding is that voter turnout was quite low among eligible voters during the first decades of the United States's existence. It rose dramatically around mid-century after the extension of universal male suffrage and the rise of mass political parties. US elections of the late 1800s were notorious for having extremely high (white male) voter turnout, generally over 70%, because of influential social and political institutions that we would probably describe as corrupt. To simplify matters, your local party organization would sign you up if necessary and get you to the polls, perhaps more than once, maybe ply you with liquor, threaten you if you didn't vote their way, or offer you a job if you did. Exactly how these political machines worked differed from place to place, but it was generally not something we would recognize as standard election day practice. The reforms of the Progressive Era -- the secret ballot, instatement of voter registration requirements, and so forth -- drove down voter turnout around the turn of the 20th century, as did discriminatory laws aimed at African-Americans and poor citizens.