I understand it all magnified as firearms became more widespread, but no nation would go "welp guys we're not gonna use metal armor anymore", since metal armor also provided some kind of defense against firearms.
Some type of metal armor has pretty much always been in use somewhere, in some quantity in most wars. Until very recently U.S. army helmets were mostly metal. They've since switched to mostly Kevlar, because the improperly padded metal helmets were causing fatal head injuries.
Some units experimented with chainmail body armor, but not for long since it was terrible. Heavy, ineffective, and turned you into your own pipe bomb.
Some tankers had chain mail on their (not metal) helmets to try and stop shrapnel injury
Cavalry were the last to abandon the cuirass, probably because they had the highest chance of close melee combat. (Also why they had sabers.) And obviously weight is less a factor if you have something to ride on.
There are many examples of cheap cast iron breast plates used (briefly) during the American civil war. There are many because they were frequently ditched on the side of roads while marching. This suggests they really weren't that useful, and of course had zero chance of stopping a bullet.
The vests sold quickly in the early days of war, but as the war dragged on the soldiers began to realize this was not a good idea to wear them.
If you check /r/artefactporn you'll see a submission in the last couple months reposted every three months of a Napoleonic era cuirass with a cannon hole in it. As a side note, in that example you see the entry hole in the front and the exit hole in the back, because it was a fucking cannon ball and a bit of meat won't stop that. However, if you were particularly unlucky, and got hit with a lesser projectile, it could penetrate the front plate and your body only to bounce off the back plate. Think "meat grinder."
TL;DR
Soldiers don't like carrying extra useless crap. Especially if the best case scenario is that it doesn't work well, while the most common scenario is that your faulty equipment actually injures you more.
Edits: terrible grammar/spelling, I blame phones.
