Nobles were required to provide troops and arms, and be skilled in their command and supply - this was far more important than their combat abilities, and why noblemen had so much power, wealth and prestige.
What happened was the emergence of the modern standing army in the fifteenth century, notably the Ottoman Janissaries and the unorthodox army led by commoners in the service of Charles VII (including one Jean d'Arc and La Hire.) Before then, in between conflicts, the nobles would return to the running of their estates, their troops dispersed back to their ordinary life. A standing army never disperses, but trains full time and is ready whenever the monarch desires to go to war.
This lead to the creation of the Spanish Tericos in the sixteenth century - a professional standing army in the service of the Spanish Habsburg empire, commanded by veteran soldiers loyal to the King rather than to the Nobility. They were so successful, the model was eagerly adopted throughout Europe, ending the time where monarchs depended upon their nobility to provide martial muscle. The trend would continue and intensify as the old feudal system began to give way to the modern nation-state - the monarchies in Europe gaining more absolute power and greater wealth to afford mercenaries and royal armies. By the time of the 30 Years War in the mid-17th century, it was almost entirely fought by standing armies rather than feudal levies.