Spartan army 400 - 390

[Aryaman2]

Fashion and utility. There is a parallel to the lose of the cuirasse by early modern cavalry (a fashion I have seen attributed to the men of Gustavus Adolfus imitating their king who had a wound from a Polish musket that made wearing armor difficult. Just as likely that his men could not afford cuirasses.) Perhaps more important was the popularity of a variety of irregular light cavalry: bosniaks, hussars, etc. After a long period of loss, the cuirassier would reemerge as the preeminent cavalry of europe.

.

Going a littlte out of topic, I disagree in some details, the cuirassiers of Gustavus Adolfus times wore the lobster half armour, that never returned. Also, I would say that the preeminent cavalry of Europe to reemerge were lancers.
Going back to the topic, the main factor is the kind of war you are fighting. Armour is fine for short campaigns with pitched battles. The longer the campaign. the marches, the skirmishes, the more encumbrance armour gets. However, if the war gets very long and armies become very professional, armour returns as soldiers get servants to carry the armour.