Such a difference between the three major cities positioned as national centers and the rest of the cities is believed to have been deliberately brought about by the bakufu's policy to secure the shogunate system, by concentrating the center of each domain into a single castle town, and by restricting the free economic expansion of the city beyond the sphere of the domain.
幕藩体制維持のため各藩の拠点は一箇所の城下町に固定化され、更に領外への経済圏の自由な拡大が制約されたため、全国的な拠点とされた三都との間に格差が生じたと考えられる。