One factor why sitting in the seiza style became popular in the early Edo period was that when the Edo bakufu (Japanese feudal government headed by a shogun) adopted the Ogasawara school of etiquette, the Daimyo (feudal lords) were required to use seiza style when they all gathered to meet the Shogun under the Sankinkotai system, which required them to spend every other year in Edo, and the custom then spread to their domains.
江戸時代初期、正座の広まった要因としては、江戸幕府が小笠原流礼法を採用した際に参勤交代の制定より、全国から集められた大名達が全員将軍に向かって正座をする事が決められ、それが各大名の領土へと広まった事が一つ。