The simple answer is technology. WW1 was a turning point for camouflage being used as a legitimate deception tactic due to advances in technology. Prior to that, in general, camouflage was unnecessary and often not even considered. Small arms and artillery had limited range so you were either in range, in which case camouflage is irrelevant, or out of range, in which case you might not even be on the battlefield.

There is also the consideration of ethics in warfare prior to the 20th century. The purpose of camouflage, to give on an advantage by concealing position, would have been considered a guerrilla tactic and therefore “ungentlemanly”. This notion was prevalent during the American Revolution, where many of the continental soldiers were veterans of the F&I War and learned their tactics from the natives (this is not unlike the shock to the US doctrine when urban warfare peaked its head during Vietnam then became the norm during OIF/OEF). This kind of warfare at the time was considered terrorism and not really warfare at all. However, there are many caveats to this idea