I just have to say that it's a poor assumption to make that a man cannot write a woman well, it's biased and pointless. Even if it's not as common, it's very plausible for a number of men to understand how women work and what they can do.
Also, in the first place, Power Girl isn't just about a girl being powerful, but it's about the character herself and her story and her conflicts, such and such.
It's like assuming a gay main character would have a title that's all about him being gay, and to say nothing about his personality and life conflicts.