@benroyce is probably the best person to comment on this, but I'm not sure it's just that people "couldn't be bothered”. There was an epidemic of #ToxicIdealism spread perhaps primarily by the “G-n-c-d- Joe” brigade leading a non-trivial number to not vote as some sort of masochistic protest.
The left wing must, finally, learn the lesson:
Fight in the primaries, unite in the election