Here is perhaps a sequence of events that lead to the closure, as well as some motivation behind it:
It has been decided by the TeX.SE community to host "Answer the unanswered" sessions (see Regular “Let's deal with unanswered questions” chat meetings?) in an attempt to reduce the number of unanswered or open questions. These "janitorial sessions" are there to refocus from the perpetual incentive to answer the next, new (perhaps easy) question, while older (perhaps more difficult) questions tend to be left unanswered and turn stale.
Another reason here is that some answers may never receive successful answers, while others are just too general to provide an effective answer. Even a recent discussion on meta.TeX.SE (see Should I self answer my question?) discussed whether specific answers to a general question should be answered in "combined form" to provide a definitive "answer to the original question".
While yours most likely did not fall in any of these categories, comments suggest that there was not enough detail to adequately answer the question. As such, it was brought up during the most recent "Answer the unanswered" sessions in chat, where your question was mentioned as perhaps being too localized/TL.
Your comment rebuttals suggest there was no need to supply a so-called minimal working example (MWE). However, this is a general misconception when it comes to answering questions on the Stack Exchange network. Not only are there a myriad of solutions to the warning "Underfull
\hbox" (see No line here to end for one cause) and/or "Underfull
\vbox", you are after solving a specific instance of this warning. This could depend on the code that you use, the document class that you use, the structure of the code that precedes that actual warning, or perhaps something different altogether (like TeX's line-breaking routine).
As a questioner you should try to put yourself in the shoes of the possible answerer in order to have the best shot at finding an optimal solution. That is why we usually push questioners to "provide an MWE that reproduces the problem." This requires you (the questioner) to provide as much information as possible so that an answerer can reproduce the problem (independent of the questioner), isolate what causes it, address possible alternatives and provide a solution.
Of course, if confidentiality is at play, 100% reproduction is very difficult. However, if the question is left in this general or even specific, open-ended setting, with no reproducibility, and it turns stale, it is most likely destined for closure as being "too localized" by definition:
This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet.
As an example, the generality of your question may just as well have been solved by (to name a few when searching for "underfull vbox"):
But without knowing more detail, a solution provided at the above links might have helped, or not.