Just to give some statistics, right now on Stack Overflow there are 8,792 questions tagged drupal, including the closed but not deleted ones; of those, 812 questions (9.24%) don't have any answer.
On Drupal Answers, there are 10,501 questions, including the closed but not deleted ones; 1,664 questions (15.85%) don't have any answer.
This could mean that on Drupal Answers there are more questions for which it is not possible to answer, or there are more questions that have been closed before anybody answered.
If I search on Stack Overflow for not closed questions tagged "drupal", I get they are more than 5000, without to get an exact number. The same happens if I search for not closed questions on Drupal Answers.
The statistics would be more precise, if I could get the exact number of not closed questions. Considering there are users who ask the same question on both the sites, and the Stack Overflow question is migrated on Drupal Answers (when they are found) where it is closed as duplicate, I imagine the statistics would be pro Drupal Answers, when excluding the closed questions.
If we look at the number of questions without an accepted answer, or without answers with a score higher than zero, then we get the following data:
Those numbers include the questions that don't have any answer, but also those questions with answers with a score of zero, or that have not been accepted. Accepting an answer just means the OP found the accepted answer more useful than others; it is quite subjective, and it is not a measure of the answer's quality. I have seen accepted answers with a negative score, which means the community didn't think the answer was a good one.
Drupal Answers is a site specific for Drupal, and questions about Drupal are not mixed with other questions; this means a Drupal question has more visibility on Drupal Answers. As a matter of fact, right now there just a Drupal question on the front page of Stack Overflow. This could not explain why there are more answered Drupal questions on Drupal Answers than on Stack Overflow, but for sure it is something that makes the difference.