There are many reasons for this, and it really depends on the type of question that is asked. Unless someone wants to accept an answer but is really struggling to do so because the UI makes it very unclear, I don't know if any changes are required. I have asked a question in Meta previously for ideas and suggestions but didn't really receive any responses: How to encourage users to accept an answer to a question?
However, just to provide some more context for this question, I'll offer some reasons that I have seen which contributes to the lack of accepted answers.
- There are many one-off questions from people who are struggling with a particular design issue, and they are just looking for another source of information or solution to the problem, and once they get what they need, they don't come back again. There is no incentive for them to mark the answer as accepted since they have no intention of returning (for whatever reason).
- There are questions that are difficult to answer, and it is often difficult to get a satisfactory answer (for whatever reason). The person who asked the question may choose not to accept any answers because there is no clear answer that is acceptable.
- People forget all the questions that they have asked, and the amount of time it sometimes take to get an answer from the community. I have been trying to go back to all the questions I have asked before (339 of them!) to mark as many as I can that I feel is acceptable, but even that takes quite some effort.
- There is a bounty system which can incentivize the community to provide answers and encourage the person posting the question to try and find an acceptable answer, but I don't know if UXSE provides enough value of social proof that people put a lot of effort into this (I have collected 26 bounties but so what?)
And this actually goes back to the heart of the problem: is there enough incentive for people to accept answers, especially when it is not necessarily the answer that they want?
I hope this helps.