There are many things in this world that follow a traditionalist way of life. Not just Drum Corps. I believe there are deeply rooted traditions coming from the Boy Scouts. Boys join the Boy Scouts and girls join the Girl Scouts. I would believe there would be a problem if women did not have a chance to join other Drum Corps, but there is a plethora of corps.
I am sure there are many organizations that do not allow men in them -- such as the LPGA (ladies professional golf association). Just a comment, women can plan in the mens PGA.
My bottom line answer is that they are traditionalists.