I find a lot of the time users who don't accept answers are usually new(er) users. They don't know that the accept option is there or they just don't know how to do it. I know this because a lot of the time I'll add a comment as such:
Glad to help! If this answer helped you out you should accept it!
And they're comment back telling me they didn't know about it.
A lot of the time I notice they will add comments like:
- "Thanks, this helped"
- "It's working now, thanks!"
- "Just what I was looking for! Thanks!"
- "Thanks Pieter B! That did just what I was looking for :)"
If key words such as those are detected, there should be a small one liner telling them they should consider accepting the answer or maybe even go so far as to add a checkbox or button next to the comment to allow them to accept an answer. Maybe an illustrated example will help:
I know it seems like a small change but I think it'll help newer users learn about accepting answers.
Also, the accepting link points to How does accepting an answer work?