Ive read the manga. Zenitsu, Inoskue, and Tanjiro are never accepted into the ranks of the hashira. It explains that you have to be the highest Demon Slayer rank, Kinoe, and have to have killed one of the twelve moons in order to be accepted into the hashira. Tanjiro and the gang did kill upper moon 6 but they were not the highest rank of the demon slayers.
