Armour did not completely disappear until the 19th century. By then only heavy cavalry had cuirasses but it shows that armour was still very much in use by most of the nations in Europe. A well made musket could easily (in an ideal environment) punch through armour. And to equip a whole army with effective armour is very expensive, especially considering the sizes 18th-19th century armies.

Armor that was strong and thick enough to stop a musket ball was far too heavy to wear into battle.