The historical panorama, presented as musical theatre, has a gestation, and impulse, that is entirely home grown. That is always tied, somehow, to American history. And, I believe, a seminal influence so subtle and indirect that you would hardly think to credit it at all. At the same time, you must provide a varied, cohesive score that is emotionally, dramatically and musically gratifying. An almost impossible task, because of the economy of musical theatre.
Song time moves more slowly than dialogue time.
And let's not forget choreography, an essential part of any historical-musical tableau. Set aside intermission and any other part of the theatrical experience that isn't actually performance. Figure your actual playing time is two hours and fifteen-twenty minutes, tops. Your time to establish clear, complex narrative is no more than half to two-thirds of that. And for many years, they wouldn't.