I don't think gender really comes into it (except for corner cases, maybe, but that's an exception).
A lot of players just tend to always think of themselves as players and never really think of putting themselves in the GM seat, or are too worried they won't be able to be a decent GM. Applies to both genders.
This seems to be my experience as well. The ratio seems just about the same in my social circles as to Girl players/girl GMs compared to boy players/boy GMs. The fact there are fewer girl players in general is the root cause of a perceived lack of girl GMs. Figure out how to get more girls into the hobby and you increase the number of girl GMs.
Easier said then done, but let's face it, this is the sort of thing that either is handled over a generation or two by raising your children with an appreciation for the nerdier aspects of society, or is done all in one huge tidal wave because of some massive media sensation that made a fad out of some Roleplaying game. The latter gets results sooner but effectively drops off quickly as well and won't stick as well, and also really is hard to predict what can do such a thing.