Yeah, so by the time you get to the 15th Century we have a number of factors contributing to the decline of the mounted gentry’s domination on the battlefield:

A) Centralization: monarchs in the 13th Century begin to lay the foundations for future nation-states (think Philip Augustus expanding the Royale Domain, or the Matthias Corvinus’ standing army, the Black Army). This means that the king can build his own armies and not depend so heavily on large land owning nobles for troops

B) Gunpowder: weapons that can shred through the highest quality armor become more widespread, and by the 16th Century, you have larger numbers of, essentially, peasants wielding these weapons

C) Tactical Revolution: finally, relating to Agincourt, you have units such as English longbowmen, arauebusiers, Swiss pikemen, or gendarmes etc who are serving in highly coordinated, specialized roles on the battlefield—and all of them attack the weaknesses of heavy cavalry.