I'm not sure that's the only (or even most apparent) conclusion. Maybe voters don't think their vote can make a difference, feel adequately represented by eithe

I'm not sure that's the only (or even most apparent) conclusion. Maybe voters don't think their vote can make a difference, feel adequately represented by either major candidate, feel the results don't have much impact on their lives, can't get to the polls (for any of lots of potential reasons), or aren't registered (also lots of potential reasons).