It's good question to ask.
From my limited perspective, i would say the people who don't vote include:
1. people who don't like either major party candidate, and think it's a waste of time to vote for the Green or Liberatarian. In effect, not voting is a protest. I believe George Carlin falls into this group.
2. people who are indifferent, sometimes violently so. They just don't want to hear about politics. Don't want to know who their congressman is. They just want to live their happy, comfortable life, without having to worry about "all that political stuff"
3. people, who for whatever reason, like work, just can't make it to the polls that day. (not too many people fall into this group, but there are probably a few)
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