you should clearly be an EA or Activision CEO because you clearly know more about potential markets and customerbase than anyone in the board of directors and management in those companies.
It's cruel business, sorry. They have limited resources. If they analyse "we can make this game strictly for teenage boys and get gazillion dollars or make it more for everyone and make only a gorillion dollars" what they would choose?
Sorry, but people who have no business insight whatsoever claiming they know better than people who are responsible for the company's wealth, always irritate me.
Long story short, there are PROBABLY not enough female hardcore gamers who want to kill everything that moves for them to care.
If it was really that profitable, some big player would already claim that slice of a pie. And if 90% of LoL userbase are men, a game that is not really aimed at men, being pretty much unisex, it tells something more than "well, his girlfriends wants to play FPS games so probably 90% of females want that too"