Because they were too inexperienced or low in rank for the cavalry, foot soldiers came to be called this
During the Ancient and 
Middle Ages Infantry were categorized by the types of weapons and armour they 
used, ... who were too-inexperienced or too low in rank for membership to the 
cavalry... [DOC] [TLE] Infantry | Define Infantry at Dictionary.com [PAR] Infantry definition, soldiers or military units that fight on foot, in modern times 
typically ...