That they were sent so far from Edo was due to Ieyasu's deployment policy to 'deploy fudai no sho (military commanders that were hereditary vassals to the Tokugawa family) on the border where enemies would make their attack.'
江戸から遠くなっているのは、「譜代の将は敵が攻めてくる国境に配置する」との、家康の配置方針による。