Why didn't the king enjoy absolute power in the Early Vedic Period?
The Early Vedic Period (ca. 1500-1000 BCE) did not see a unified kingdom, therefore no king enjoyed absolute power. The first Vedic "state" was the Kuru Kingdom which appeared during the Middle Vedic Period (ca. 1200-850 BCE). This was a collection of many different tribes forming together and mixing their prior customs to form new traditions. The governance of Kuru involved a complicated system of hierarchal command including nobility, priest, warriors and peasants. Due to the semi-nomadic nature of the tribes, the kings used Brahmin chiefs to control the people. One tradition was the horse sacrifice where a consecrated horse was released to roam the land. Areas where the horse wandered were required to pay tribute to the king or face conquest. This unique system was highly subject to influence by the warriors and priests who followed the horse for a year. They could herd the animal according to their own desires.
The complex nature of ruling semi-nomadic tribes bound loosely together effectively eliminated the ability of the king to wield absolute power. A careful balance had to be maintained allowing Brahmin priests and tribal elders to rule their respective people while submitting a portion of their gain to the greater kingdom. Ultimately the Kuru Kingdom fell to the Salva tribe shifting power to the east into the Panchala Kingdom.