What happened was that european medieval societies had way less means of mobilizing their population to war than ancient societies. When the western part of the roman empire began do decline the cities became way less safer than they were during previous centuries, so more and more people started to ran to the countryside and isolate themselves in small rural communities.
A consul or a roman emperor could directly interact with a great deal of their population by recruiting troops in the cities, while a medieval noble or king had the population of their territory way more dispersed and harder to contact and conscript for war. So with less poor people to go the infantry, rich people were always the cavalry, there were less ways of fighting off a cavalry charge and they became way more effective.
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