Monarchs had always tied their identity to war. The European monarchies were originally the product of the Germanic conquests at the end of the Roman period, and it was a point of great pride among noble families to be able to trace their family lines back to the warlords of old. Political loyalty was to the king one served, not the territory in which one lived. Likewise, territories were won through war or marriage, so they did not necessarily make sense on a map; many kings ruled over a patchwork of different regions that were not necessarily adjacent (i.e. they did not physically abut one another; a present-day example is the fact that Alaska is part of the United States but is not contiguous with the "lower 48" states). Kings not only fought wars to glorify their line and to seize territory, but they had nobles who egged them on since war was also fought for booty. Kings and nobles alike trained in war constantly, organized and fought in tournaments, and were absolute fanatics about hunting. Henry VIII of England spent about two-thirds of his “free” time hunting, for instance.
By about 1450, military technology changed significantly. The basis of this change can be summed up in a single word: gunpowder. First developed in China, but first used militarily in the Middle East, gunpowder arrived in Europe in the fourteenth century. By the fifteenth century, it was increasingly widespread in war. Early gunpowder weapons were ridiculously inaccurate and dangerous (to the user) by later standards - they frequently exploded, they were grossly inaccurate, and they took a long time to reload. They were also, however, both lethal and relatively easy to use. It was easy to train men to use gunpowder weapons, and those weapons could easily kill a knight who had spent his entire life training to fight.
Thus, by the later part of the fifteenth century, wars were simply fought differently than they had been in the Middle Ages. There was still the symbolic core of the king and his elite noble knights on horseback, but the actually tactical utility of cavalry charges started to fade. Instead, squares of pikemen (i.e. soldiers who fought with long spears called pikes) supplemented by soldiers using primitive muskets neutralized the effectiveness of knights. In turn, these new units tended to be made up of professional soldiers for hire, mercenaries, who fought for pay instead of honor or territory.
Another change in military technology was the emergence of cannons, which completely undermined the efficacy of castles. The ability to build, maintain, and operate cannons required advanced metallurgy and engineering, which in turn required highly skilled technicians (either royal ones or mercenaries for hire). The most famous case of the superiority of cannons to walls was the Turkish siege of Constantinople in 1453, which finally spelled the end of the Byzantine Empire. The result of the artillery revolution was that fortresses and walls had to be redesigned and rebuilt quite literally from the ground up, a hugely expensive undertaking that forced monarchs and nobles to seek new sources of revenue.