The caption is misleading, to be honest. Knighthood did not disappeared, just because the knights lost their importance on the battlefield. It still exists today, most famous are surely the different orders in Great Britain today. Elton John and Sean Connery are two famous examples for modern day knights. Of course you will not see them ride into battle.
Even from the viewpoint of a military historian the exact time of the end of the knights on the battlefield cannot be defined. In most cases the invention of the black powder is seen as the main reason for the end of the heavy armoured riders in battle. But if you take a closer look you will see that this is not the truth. Weapons with black powder existed already in times that are known as the blooming period of the knights, in the high middle ages. And also in later times the cavalry forces were a crucial factor in different battles. That these riders did not belong only to the nobility any longer was not the result of the developments in weapons technology but of a new way to recruit soldiers (who in this times can really be called soldiers instead of warriors) and in battle tactics. Mercenaries became more important. However, the officers still came from the nobility. The efficency principle was introduced for the first time by Napoleon Bonaparte, a long time after the end of the middle ages. Even after the middle ages it was common within the officers to take part in the charges of the cavalry, a relict from medieval times. Some of high ranking officers were wounded or killed in these charges, a famous example is the Swedish king Gustav Adolf who was killed while leading a cavalry charge in the 30 years war.
There were crucial changes in battle tactics in the middle ages. Charges executed by the knights alone were in many cases not succesful. This was first of all the result of the increasing discipline of the infantry and the more effective tactics and weaponry. Long lances, pikes, halberds, strong bows and crossbows made the foot soldiers more effective against riders. The tactics where not so new. They can also be seen in the the battle of Hastings in 1066 in which the attack of the Norman armoured riders were stopped by the shield wall of the Anglo-Saxons. Weapons on the basis of black powder, especially those in the 15th century, were not very effective at the beginning and produced in far to small numbers. To make it worse, there was the constant risk of blowing yourself up with such a weapon.
As I said before, the hiring of mercenaries became more and more important. This lead to a professionalisation of war. These soldiers were organized in war bands which were lead by experienced captains. These groups were very organized, disciplined and well equipped. They were an important factor for the decrease of the importance of the knights in war. At the beginning they were mostly hired by the cities, who possessed a great amount of money.
The loss of importance of the knights in war did not mean that mounted warriors were not important anymore. The opposite is true. Their form changed. Armoury became lighter and more resistant at the same time, the riders became faster. Lances and sabers were the standard weapons for a long time to come, later they were also equipped with special pistols. You have to distinguish between the knight (=warrior from the nobility) and the cavalry (=mounted soldier). Cavalry existed until the first years of the second world war.