Because it works again. Soldiers largely didn’t wear armor for 300 years because there was no material light enough to carry all day and strong enough to stop bullets. Now there are several.

Unless you were of noble blood, your chances of having plate armour on the battlefield in 1500 CE were probably pretty low.

The comparison does point out the why though. Trained soldiers are an expensive resource, so protecting that investment makes sense. In a medieval battles you would supplement your expensive knights, trained cavalry, and other glorious actors on the battlefield with a common, cheap, and quite disposable resource: peons — commoners pressed into battle with a pointy stick (spear) and little in the way of armour, or, 'meat shields' — to line up and point in roughly the right direction. Nowadays that approach wouldn't do well for morale and popularity, so every soldier gets armoured up.