There was a video I had seen once about a talk given by C.S. Lewis on Chivalry. It has unfortunately been removed from the internet, but I'll summarize it here, because it was persuasive to me. It starts by examining a quote from la Morte D'Arthur, the pinnacle book of chivalry, when the French added Lancelot the knight of Chivalry to Arthur's round table.

"And thou was the meekest man and the gentlest that ever ate in hall among ladies. And thou were the sternest knight to they mortal foe that ever put spear in the rest."

-Sir Thomas Malory on eulogy of Lancelot

It's a little different then you might have expected, right? C.S. Lewis says this line says everything we need to know about Men's role in chivalry. The idea goes: If we can make a spectrum of men's attitudes, and on one side we have the softness, gentleness, agreeability, kindness, etc. and on the other side we have sternness, ferocity, aggression, dominance, etc. then the Chivalric man would exist at BOTH ends of the spectrum. He is after all, the meekest man to dine in hall with ladies, and the sternest knight to ever put spear in the rest. The key being that he is not both at the same time, but he is what he needs to be at any given time. Rather I shouldn't say that he isn't both ends of the spectrum, but that he is either end of the spectrum, based on context.

With that idea of chivalry in mind, what changed it? Well, I would break away from C.S. Lewis here, and say that attitude towards war did. When WW1 and 2 happened, the world saw a change in conflict, from war, to total war. With total war came no peace, even for civilians far removed from the battle lines. Since every man had a constant need to be wary of the enemy, the idea that a man should ever be meek and gentle fell to the side. Instead he had to be steadfast and ready for action at a moment's notice. The change in attitude killed the Chivalric man, and since war is a man's endeavor, I see it fitting to say that men killed chivalry. Nowadays, you might even have read the description of the two ends of the spectrum and thought that the softness, agreeable end would be more feminine, and the other end more masculine. However, I have a word of hope here. With the recent movements for men to get more in touch with a gentler side, I believe we are living through a renaissance of chivalry, and that we all would be better for it.