Or they lived in some of those insane regions where only a single voting office served thousands of people and couldn't affording standing outside for 8+ hours, or they couldn't get a tuesday off (sane countries allot a whole weekend for voting), or they couldn't afford to get sufficient id to satisfy their state's demands. (Yes, i know, mail-in voting is a thing, but i'm sure there are good reasons to keep people from using that as well.)
There's plenty of reasons otherwise eligible voters didn't vote beyond "didn't care" and many of them are "they were prevented from doing so by institutional problems".