My original conjecture was because the armor wearing classes began talking to the armorers. She wasn't satisfied with that, so we argued a bit, and hit on a revision of the conjecture.
We propose that Europe had developed a mercenary middle class due to its ongoing wars. For a farmer/soldier, investing in armor that would be used sometimes, but not often, didn't make much sense. However, for a professional soldier, better protection meant everything. These people risked battles every year. No armor was "good enough." If they had extra money, they would invest it back into their kit. It was these mercenaries that pushed the envelope on armor technology, constantly striving for better, more comfortable, more durable, and more practical. As they pushed the armor technology, so the rest of Europe followed.
Why didn't the sumptuary laws limit the armors of non-nobles? If clothing could be regulated, then surely armor should have been regulated? There are a number of factors preventing this. First, your enemy is always looking to hire the best armored troops that he can hire. You have no incentive to limit the armors of your non-noble troops because you'll be outclassed. Nobody held enough of Europe to put any restrictive laws in place, so mercenaries would always come from somewhere. Second, although the armors were expensive, they weren't sumptuous for the average soldier. A rich suit of armor said that you were a noble, and the enemy would seek to kill you, capture you, and hopefully hold you for ransom. If you're an ordinary mercenary, they'll figure out that you aren't worth anything and get mad, likely killing you. On the other hand, as an ordinary mercenary, the enemy might just let you go if your side lost the fight. Any noble who butchered mercenaries would soon find himself unable to hire them, so its in the winner's interest to let mercenaries leave the field, or even hire them on.
Yes, soldiers could fight to the death, but usually they didn't need to.
How do demonstrate this conjecture? That would take work. I welcome anyone to do so.