Armour is expensive, heavy and largely ineffective against firearms at the time. Keep in mind these were armies trained by western officers in western tactics. The time of wearing armour was since past in European and American armies.

Common soldiery wasn't considered particularly valuable. The best part about guns was that you could pick up anyone off the street and they could be an effective cog in the killing machine. Powder, patch, ball, bang, repeat. There wasn't the years of training required to master archery or a sword. Just a few weeks to drill them in reloading and formation. So losses were easier to accept and replace.

Even primitive firearms were effective against plate armour designed to fend off swords and crossbow bolts. As firearms grew more sophisticated, the sheer amount of metal you'd need to wear would be ridiculous. Heavier armour means your troops can't march as far nor as fast. The last troops in european armies to wear armour were pikemen and cavalry. Usually a breastplate and a helmet. These were largely for when they closed to fight hand to hand with the enemy and not against firearms. As pikemen were phased out and cavalry started using carbines, the use of armour was discarded altogether.

When Japan was first training it's modernized armies they hired forign experts who passed along these few decades worth of lessons learned by other nations fighting on battlefields dominated by gunpowder. The common wisdom was that it was unnecessary to give common soldiers armour because it was an increased cost for little real gain and the individual infantryman could be replaced by fresh recruits and the deadliness of firearms made it all but useless.