Sometimes the overhead of that maintenance work isn't worth it, particularly for a bug that's been explicitly categorized as 'low priority' for years. The fact that a bug can stay at low priority for years is a reasonable sign that it's not too important to spend a lot of effort either tracking it or fixing it.
With that point of view, closing the defect as unfixed and explaining the reasons why is in some ways more honest than just keeping the thing open ad infinitum just because somebody, at some point in the past, thought there was an issue. And if closing it was wrong, you'll run into the issue again, and can either re-open the defect or submit a new defect with a back link to the original, and an explanation of what's changed that's brought it back into relevance.
(Personally speaking in the last few months, I've gotten several good solutions from reading the commentary on defects that were closed as unfixed.)
