Some reasons why I don't vote.
- I don't vote on duplicate content.
- I don't vote when the question/answer is poorly written or formatted.
- I don't vote when the issue is overly specific and does not contribute meaningfully to the broader community.
- I often do not vote when there's already an accepted answer.
- I don't vote on questions or answers that are readily available in official documentation.
I basically see voting as recommending an answer to a user where the question has been unanswered.
Also I tend to spend my efforts trying to answer unanswered questions. As such, voting in this context is often not required.