A question I just stumbled upon, Why is the ending of Death Note in the anime so different from the manga?, is an example of the generic question type: "Why did they change X?" or "Why did they write it that way?". The only person that can answer these questions are the writers, resulting in any such question being unanswerable if there isn't an answer posted somewhere on the internet. As is outlined in the FAQ of every Stack Exchange site,
You should only ask practical, answerable questions based on actual problems that you face. Since the vast majority of these questions won't be answerable, they should all be closed. After all, any answer not based on an official statement from the author would be 100% speculation.
I'm sure someone will raise the objection that since some of these questions have answers somewhere on the internet that those ones should be opened. However, since there is no way of knowing if such an answer exists until someone posts it, then that means we can't know if an answer is answerable when the question is posted. If we can't know a question is answerable, then it's unanswerable.
For the sake of argument, let's assume we did allow these questions when an answer from the author exists. Since we still can't know if the question is answerable when it's posted, it means we would either need to:
- Assume an answer exists until the question goes unanswered for a certain amount of time.
- Assume an answer doesn't exist until someone finds it.
Neither of these situations is acceptable. Assuming an answer exists for a set amount of time will result is likely to have the question be forgotten about, and never closed (clogging up the site with questions which should have been closed and deleted). In addition, any time we chose would be completely arbitrary. Do we wait an hour? A day? A week? If we need to start making arbitrary rules up to allow a question that's a clear indication the question should not be allowed.
Assuming an answer doesn't exist means we'd need to close the question, and then leave it on the site in a closed state indefinitely, until someone posts a comment saying they found an answer. Frankly, this is a completely ridiculous course of action to take. Having a bunch of closed questions floating around is just as bad as having a bunch of opened questions which should be closed.
Furthermore, closing is meant to be a temporary state, where all questions will either be deleted or re-opened (in the latter because the author fixed them). Closing isn't intended to be used on a question we never have any intention of re-opening unless a specific event eventually happens. After all, if we're just going to keep the question around forever, what's the point of even closing it in the first place?
In conclusion, "Why did they write it that way?" questions should be deemed off-topic since they are not answerable.