This is more of a general comment - rather than related to the specific question linked in the post. But since several comments revealed that some perceived problems were actually caused by lack of understanding how (rejected) migration works, I decided to post an answer anyway.
This particular question has been deleted in the meantime. (And the closed copy on Mathematics Stack Exchange will eventually be deleted too.) That means that these links will eventually be only visible to 10k+ users - but still I'll add link to the revision history and timeline of the MO post, and revision history and timeline of the MSE post.
The one posted on Mathematics can be found in the Wayback Machine. I did not find a Wayback Machine snapshot of the MathOverflow version.
What happens if a question is migrated and then closed
When looking into anything related to migration, it might be useful to have a look at the faq post: What is migration and how does it work? (Or at least at the parts which are important for the problem you're facing.)
In this case a question was migrated to another site and then closed - so the section "What causes a migration to be rejected and what happens after?" is relevant here. See also the corresponding tag-info on Meta Stack Exchange.
Whenever a question is migrated to another Stack Exchange site and it is closed there for some reason other than duplicate, then the question is returned back to the original site. (There are some exceptions - if you're interested in more intricate details, you can find them in the linked post.)
In this specific case it means that MSE users did not have an option to close the question without returning it back to MO - unless there was a suitable duplicate target on that site. In particular, this means that the suggestion given in one of the comments - that the MSE users should have closed the question without returning it to MO - isn't technically possible.
I will explicitly mention that there is a feature request suggesting a different behavior: Fix the counterproductiveness of migration rejections by eliminating them. Although personally I do not agree with that particular feature request. In any case, this is how the Stack Exchange software works at the moment - so the only option we have as the users is to take into account the limitations of the software.
What can be done next?
Actually I do not think there are really that many options.
- If you think that the question is suitable on MO as it is, suggest that it is reopened. I.e., vote for reopening - that will put the question into the reopen review queue. And if the votes from the review queue do not help to reopen it, you can try to make a reopen request in the designated thread here on meta - which has the additional advantage that one can explain why they think the question should be reopened and a further discussion can be held in the comments. (However, this doesn't seem to be a suitable option for this specific case.)
- If you think that the question is suitable on MSE as it is, suggest to the OP that they might repost the question there. If there are some changes, which would improve chances that the repost would be received well on MSE, suggest them to the OP in comments. (But in this case, the migration was already rejected. So it doesn't seem very likely that the question would be received well.)
- If you think that the question can be improved to be a reasonable MO question, you can suggest the improvements to the OP in the comments.
- If you think that the question could be changed into a question that would be welcome on MO (or MSE), but you don't think that the OP has sufficient knowledge to formulate a good enough question, you can post the question yourself. (In such case, it would certainly be polite to mention the original question and include the link - even in situations when the linked question is only visible to 10k+ users.)
Rules about allowed questions can be interpreted in many different ways
Different people can judge suitability of a question for MO or MSE differently. (To see some examples of this, it suffices to have a look at the recent close and reopen votes reviews - one can see that some reviewers did not agree with the original close/reopen vote and they voted in the review differently. Users with sufficient reputation have access to full history of recent reviews: closed votes and reopen votes.
Other users can get some information about recent reviews using SEDE queries, such as this one.) So luck might be a factor, too - it is possible that the question which was deemed unsuitable for MO by some users, would be well received when judged by some other users.
This is, to some extent, natural - even if the site has very specific rules on what is and isn't acceptable, various people will understand those rules differently.
So maybe "I know it when I see it" is a better way to describe how people decide whether a question on MO should be left open - rather than checking some extensive set of rules and guidelines. For example, one can find on MO some questions suitable for the students starting studying at the university level - despite the fact that the site should be only for questions above a certain level. But still, if MO users find such a question interesting, they might decide to keep it on this site.
I think this Todd Trimble's comment describes it quite well: "I think one of the main criteria for "success" at MO is whether the users find a question interesting or challenging; that in itself serves as "self-motivating". Thus lack of an explicit motivation or setting of context is often seen as forgivable, if the question is perceived as having apparent intrinsic interest already."
Soft questions on MO and MSE
One should keep in mind that the sites change over time - it is natural that maybe some types of questions which were accepted on a site when it was small are accepted more reluctantly after the traffic gets bigger. (Basically the same thing was pointed out in David Roberts's to another recent question: If it doesn't interest you, don't read it!)
In particular, I'd say that both on MSE and MO, soft questions (and big list questions) were more welcome when the sites were relatively new. (You can find some older discussions related to soft questions on this meta, such as: Crazy idea about soft questions.)
One can check that the questions tagged soft-question had a better chance of succeeding in the early days of this site - and the same seems to be true about big-list. I have already mentioned some stats in another discussion: Why are some non-research questions so highly upvoted? - some of the