Why are some Stack Exchange sites so inactive?
I believe one reason is that is the script that automatically deletes questions aka Roomba. Currently, that script deletes questions with no answers, score = 0 or below, no more than 1 comment, and not many views. This means that the script will delete a large percentage of questions on small sites, unless there are some active user(s) answering questions. E.g., on Stellar most questions will get removed (even without getting a downvote).
Posting a question knowing it'll likely get removed is not motivating. Also, the deletion of good questions also causes fewer users, resulting in fewer votes/answers and overall lower site activity (e.g., potential new SE users who could have found the question via Google won't be able to find it if the question was deleted).
Therefore, I recommend to decrease the required view count to prevent Roomba deletion when score = 0 on smaller sites. E.g., Roomba’s strict auto-deletion criteria are considered problematic in Arqade SE.