Always more to be said on this topic but you might be interested in this post by u/WARitter as well as this post by u/WARitter and this post by myself.
Armor didn't become completely useless, it still protected against swords and pikes and might stop pistol bullets or ricochets to varying degrees, but the growing popularity of more powerful muskets and declining quality meant that it was generally losing its arms race and was becoming much less important in many military applications.
In the second half of the 16th century, many military thinkers began reccomending that foot arquebusiers and musketeers wear no armor at all, except for perhaps a helmet. The problem with arquebusiers even wearing light mail as explained by William Garrard in 1591 was that:
by that time they haue marched in the heat of the Sommer or deepe of the Winter ten or twelue English miles, they are more apt to rest, thè„ readie to fight, whereby it comes to passe that either the enterprise they go about, which requires celerity, shall become frustrate by reason of the staie they make in refreshing themselues, or else they are in daunger to be repulsed for want of lustines, breath, and agilitie.
Sir John Smythe pointed out that fighting as an arquebusier could be deceptively tiring. Already he was pretty heavily laden with his gun, sword, ammo, and other equipment and delivering volleys with a countermarch drill would require constantly marching back and forth while reloading his heavy weapon. In a skirmish especially an arquebusier needed to be constantly stooping, crouching, and dashing from cover to cover while fighting.
By the end of the 16th century skirmishing, raiding, and "small war" was increasingly becoming the main form of combat and even it pitched battles it had become typical for the fight to be decided by gunfire well before the infantry ever came to a push of pike. Military thinkers still considered armor very important for melee infantry and cavalry, but the soldiers themselves often preferred to wear less and less of it to reduce weight and discomfort. Sir John Smythe had a very negative opinion about the practice of english soldiers discarding their leg and arm harnesses to reduce weight, but still felt that footsoldiers shouldn't bother with pistol or carbine-proof armor since he felt it would be too heavy. In 1683, James turner wrote that the main excuse given for the lack of armor in modern armies was that "the long and continuated marches of our Modern Armies, not only for many days, but for many weeks and months, both in the extream heat of Summer, and the rainy and tempestuous weather of winter, require that the Souldiers should be eased of the weight and trouble of their Defensive Arms".
Turner suggested that armor might be made feasible again with training, pointing to examples such as the ancient Romans, but claimed that soldiers were no longer willing to wear armor often enough to get used to the weight.