Body armor was altered with the advent of firearms, certainly. It did exist through both world wars an into t modern era, though was not commonly in use during the Great War and WW2 for multiple reasons (there are debates as to the weight each reason had).

Efficiency: increased weight means reduced efficiency, a trade-off which most deemed unworthy

Comfort: Armor, particularly plate armor sufficient to stop bullets and shrapnel, is not terribly comfortable

Material cost: during the world wars, many soldiers found themselves without the basics of clothing and food. Armor was not prioritized as highly as those things

Carry Capacity: every pound of armor a person carries is a pound of other gear that must be sacrificed. This falls hand in had with efficiency.

I'm sure there are other reasons, but these were significant, at any rate.

Recent years have supplemented plate armor with modern materials like Kevlar and and advanced plastics which have swung the scale of efficiency vs weight, cost and comfort back in favor of wearing armor. That coupled with more modern transportation methods which have increased comfort and reduced the foot miles of soldiers has made it more worth the displeasure of wearing armor than carrying the risk of death... Though every soldier I know who has worn the stuff in the 'sandbox' curses the miserable heat of them, a problem that was no doubt shared with those who wore it 100 years ago.