I pondered this question as I tried to develop questionnaires and a methodology to measure this phenomenon. It is very hard find out from survey research why people don’t do something.
Not surprisingly, when I finally figured out the right series of questions and the right methodology I was able to figure out the answer. Sometimes when you know the correct question and the correct methodology you know what the answer is.
What I developed was a series of self addressed postcards sent to me by a split sample in which my address showed me as part of a variety of organizations and the postcard had a variety of reasons the respondents explained why they were planning to vote.
The answer was that people who don't vote: do not consider the political system surrounding their life to be inadequate. When it becomes intrusive or troublesome they will turn out to vote. Most of the time they are not dissatisfied.
I myself decided to go along, for many years, without voting in order to think about ways to harness this non-voting population into a voting block. If even a small percentage of non-voters in any election could be mobilized they would determine the outcome of the election.
I never conceived a way to harness this population.