I don’t have a complete answer, but the mechanization of warfare should have a lot to do with it. Things like the semi-automatic pistol – they would have called it an “automatic” pistol during that era – the modern hand grenade, the airplane for reconnaissance (although lighter-than-air balloons might have been used? IDK), and other weapons wouldn’t have really appeared for the Mexican-American War (1846-1848).

Or did you mean the Spanish-American War (1898)? I think the same logic still stands, especially since train networks would have been more fully developed by the turn of the century.