Because of his trauma where he was mistreated by woman in childhood. Obanai actually disliked woman for a long while seeing them as weak it's not until mitsuri became a hashira as well did he start seeing woman in a new light. Mitsuri is strong and pretty even if she doesn't see herself as such. Obanai is the serpent hashira using a wavy like designed sword to slay man eating demons. A hashira is the highest rake in the demon slayer core making him quite powerful same with mitsuri his love