But lance was in constant use until the end of cavalry in 20th c.
Light armors (chain mails) were in constant use in Europe until at least 1775. So no. Europeans didn't forget to make light types of armours.
I suppose that it was more an idea of uniformization of cavalry than anything else, that caused disappearance of light armours. Uniformed cavalry had to keep some standards, described in regulations in detail. Those regulations were written by state officials. It was the state which paid for uniforms and weapons, so there was no place for an individual choices.
Every armour meant some additional cost. As long as weapons and ecquipment were buying by individual soldiers, there was a tendency to pay more for personal protection (different kinds of armours - more comfortable light ones and less comfortable heavier ones). When states began to provide weapons and uniforms, there was not so much pressure to protect soldiers. Better protection = higher costs. So states divided cavalry into 2 categories:
1. those ones who still used armour - and this category preserved plate armours (plate armours gave better protection against bullets and blows at lower costs, but were less comfortable than lighter/elastic armours)
2. the rest - unarmoured
There was no place for chain mails, which were sometimes even more expensive than plate armours, but didn't protect from balls. The advantage of chain mails was a comfort of use, but a comfort of soldiers was the last thing states took care

.