In a word, there were many turbulences in various districts before Yoritomo formed influence in Togoku (the eastern part of Japan, particularly Kanto region), and it led to attrition of the districts to suppress by dispatching Togoku Samurai (a group of samurai in the eastern part of Japan).
即ち、頼朝が東国に勢力を形勢する以前から諸国では、騒乱も多く、その度に東国武士を派遣して鎮定することは諸国の疲弊にもつながる。