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?