Out of idle curiosity, I was looking through some of my (old) answers to questions which have been reasonably upvoted, were often the only answer, but have not been accepted.
Fair enough, you could argue that some of those answers fell short in some way, but on further investigation it seems there are a number of users asking questions who hardly ever accept an answer (and not just by me(!); they have accepted answers for <20% of their questions). For the record - on Physics SE I have accepted answers for 19/23 of my questions that have an upvoted answer, and 4/5 on Astronomy SE (admittedly I accepted a couple of these after review while writing this question!)
Why are some users just not accepting answers? Are the answers unsatisfactory? Are users just forgetting they've asked the question? I think if it is an old question and has upvoted answers, then it might be polite to at least indicate why the answer(s) doesn't meet your requirements, especially if you want answers to any more questions.