Why historical fiction?
It puts the reader right into the narrative at the time history was actually happening. Historical fiction provides a “feel” for the place and what the events looked like to those actually experiencing a particular passage of history as it whizzed by.
A personal “lived experience” perspective to the illuminating episodes and back stories behind the headlines of the remembered historical narrative?
A chance to frame important historical controversies in the immediate context of the events that made the decision points controversial.
Historical outcomes only look inevitable in retrospect, not at the time history was forging the outcome in the fire of burning and intense current events. Looking back, everything looks inevitable. Looking forward, everything is wracked with contingency.
Story? The power of human agency to stand up to the gods of inevitability. Heroes and heroines can and do make a difference. Example is the greatest molder of future human behavior.