On answering a question that no one has asked In historical research I came across a novel question and, after investing time in it, I have an answer. This helps my original project, but the explanation to the novel question is so substantial that I would like to  publish it separately.

It is well known that X did Y. The thesis of the proposed article is that Z explains X's decision. Unfortunately, I can't point to any publication that asks why X did Y, and Z has no obvious relevance to present day issues.

Why should reviewers want to see an article addressing an unknown problem? What can I do to justify the project?
Well, this is the core of research: You ask questions due to your knowledge and expertise that others probably cannot ask. And then you publish your results. There is no general need to justify the question as long as your question makes sense. It will be sufficient if you describe how you arrived at that question. You certainly have read some literature for that, so you will also cite some sources. 

And reviewers will be happy with it if your methods are fine, your results are plausible etc.