The only thing I can proffer is that the close voters probably felt as if this is strictly a matter of style that may vary from university to university, or from style guide to style guide, with no one source being so definitive as to disallow the other possible answers. Although I do not necessary agree with this rationale, because the closure reason allows there to be for some difference of opinion provided that a sound basis for it can be supplied. That sentiment is shared by the help center's What Types of Questions Should I Avoid Asking webpage, and the good subjective-bad subjective guidance.
However, even if I were to apply the more stringent test I still agree with you. I agree with you a little too much you see. Upon trying to do my own independent research to try and explain how there might be a plurality of answers I have only really found one, and this gives the question somewhat of the opposite problem. The question is so easy to answer in its present form that it is general reference.
The very first search I performed was master's degree apostrophe and the most promising webpages I saw while performing that search were from Western Michigan University and Daily Writing Tips and upon checking those pages it seems as if they both make the same endorsement. While some of us might not think that Daily Writing Tips is a very authoritative website in and of itself, it does have some writers with disclosed credentials and more importantly it makes reference to M.L.A. style. Thus these resources represent some adequately trustworthy resources, including the university which hands out the degree, the graduates who hold the degree and a highly respected style guide. I do not really think we can add much more value to that, especially since this question is merely asking for the rule, rather than an explanation of it and we have no way of knowing why this sort of answer is not satisfactory to the questioner.
When a question can be authoritatively answered using readily available resources then the question is too simple according to the general reference test and should be closed for the following reason:
Please include the research you’ve done, or consider if your question suits our English Language Learners site better. Questions that can be answered using commonly-available references are off-topic.
And if you think that test is too stringent, as you can see from the provided "research" link, many of our members would have it so that a question that does not provide any of its own research should be automatically closed, seemingly irrespective of whether or not the answer to the question can be easily found, and the questioner has not even tried. This may have influenced some of the votes to close, but the stack exchange system is somewhat flawed in that it usually only displays one of the closure reasons, except in the case of a tie, and that is the one which receive the most votes. (The reason I call this a flaw is because it makes it harder for people to realize that a given question may have more than one problem that needs solving when trying to have it reopened.)