I think it's almost entirely, if not completely, a socialization issue. Whether people who grew up female realize it or not, they have been raised in a patriarchal culture (I know they're out there, but matriarchal cultures are few in number). In many everyday cases, women are subliminally harassed, and in some cases threatened, to not have any show of female authority, or be in a situation where they are not deferred to male occupation.
Just about all of the metalheads I interact with in real life were assigned female at birth, but I have a very hard time finding many all-female metal bands. If a female metal band contributes to metal and metal subgenres, this could reinvigorate misogynistic ideals from the male dominated metal scene, and either put the females under distinct scrutiny. This would, and does, send messages to other females/women in the metal scene that they somehow aren't worthy of contributing. These are just my thoughts, and I'm sure something's missing from them.