Armor did not disappear right away after firearms started disseminating in the battlefield, not only because it could still be useful against all other sorts of weapons like swords spears or bows, but because the response to firearms was not to drop but to develop even heavier armor, which was actually effective at stopping pistol shots and arquebus shots at long ranges. The problem was that this armor was extremely heavy, one Maximilian armor of 1520 weighed 42lb/20kg, and an English armor of 1590 no less than 71lb/32kg lb, while the armor for horse and man of the Count Palatine, 1530, totals about 120lb/54kg. As such horse armor was abandoned during the century and instead of full plate armor soldiers would use three quarters armor, which protected everything up to the thighs. This armor was used extensively during the 16th and 17th centuries, mainly by cavalry which was often of aristocratic status and could afford the expenses. The pikemen that composed the heavy infantry of the day were equipped with similar but lower quality armor, although the front ranks were often better equipped that the rest of the unit, while musketeers usually wore just a helmet. The Dutch during the wars against the Spanish also experimented with shotproof bucklers for the sword and buckler men who were tasked with breaking apart pike formation in close quarters.

So what was the problem? Well as I alluded above the price for shotproof armor was a terribly cumbersome weight, which made armor not too versatile despite their on paper effectiveness, and pikemen would often leave the detachable thigh and arm protections behind to lower the load. There was also a question of cost, as a shotproof corselet could cost a much as ten times as a normal one, so the distribution of quality armor was very uneven, as much of the equipment for war was during the period, as standardization was the exception more than the rule and as states struggled to bring more soldiers to the field in the ever increasing and devastating wars of the 16th and 17th centuries. By far the greater problem was not the cost per se, but its cost and effectiveness vis a vis what the armor faced, firearms.

In comparison to the insanely heavy and even more insanely costly armor of the period, firearms are very versatile and cost effective, which means that you can arm far more soldiers with muskets than you can equip them with shotproof armor, and the slow but sure trend during the early modern period was to increase the proportion of firearms in the army, first they were sprinkled among the crossbowmen and longbowmen, then they became the only projectile weapon on the battlefield but were outnumbered by the heavy infantry, then they equaled the number of pikemen, by the late 17th century the pikemen were the minority and in the 18th century line infantry was composed exclusively of musketeers. Not only is it cheaper but also far more scalable, which means that firearms could become stronger for far less investment than armor could, simply by making the barrel of the gun bigger. Thus the original 16th century musket, which was a bigger version of the arquebus and needed a forked rest to be able to be fired due to its weight, was the response to the proliferation of shotproof armor in the battlefield. The musket was indeed heavy and unwieldy, but not as much as and still way cheaper than the armor it was meant to penetrate. Thus the heavy armor of the cuirassiers and the pikemen was very cumbersome and expensive for very little return in a battlefield saturated by fire, and as such the late 17th century sees a shift away from armor, shedding protection in thighs and arms, and then ditching it altogether, which is also around the time in which the heavy musket is replaced by a lighter version that resembled more the arquebus of old. In the 18th and early 19th centuries heavy cavalry did sometimes retain at least the helmet and the breastplate, which were still useful in the melee against enemy cavalry, although still burdensome, but infantry completely gave up on armor in favor of musket and bayonet.

Sources:

• Keith Roberts, Adam Hook - Pike and Shot Tactics 1590-1660

• Brent Nosworthy - The anatomy of victory: battle tactics 1689-1763

• Vladimir Brnardic, Darko Pavlovic - Imperial Armies of the Thirty Years’ War (2): Cavalry