Recently, I had a question closed because it was, strictly, a maths difficulty. It occurred during my reading of a physics text named in the question. Admittedly, it was not a profound question, and I did demonstrate my effort to understand the statement, but was self-dissatisfied because my method was tedious though correct. I thought I ought have seen the answer more quickly. A reader gave me the quick method I sought, and I was pleased. Then, another reader with a high reputation commented that my question had no physical content, and closed it. I did not mind the closing of the question, but I thought the criticism was legalistic. If a reader of a stated physics text finds a maths difficulty and is discouraged from using your site, then you are not being helpful nor encouraging, in my opinion.
Your question was closed due to five close votes by site members, not by anyone in an official role, so the closure doesn't reflect any official site policy but just some of the site members' personal views. Those five people didn't include me, and indeed I would have left the question open were I asked (mainly because I didn't understand it :-). I note the question has a reopen vote, though since you have your answer it's not obvious that reopening the question would achieve anything.
I would be wary of attaching any great significance to the closure of your question. We get so many homework questions that we are sometimes a bit trigger happy when it comes to clicking the close button. Sometimes mistakes are made and we close a question that should have been left open, and sometimes we fail to close a question that really should be closed. Such failings are an inevitable result of the democratic nature of the site, but I think the site works pretty well most of the time.