Most of my friends just don't get League. It's not that they would suck at it, but I think the competitive nature of MOBA doesn't appeal to them as much. Of course I love League as a female, being a naturally competitive person. But most of my girl-friends just like MMORPG's and social games much more than these kinds of things.
Also, the community as a whole isn't the friendliest bunch, and if someone figures out you're a female things can get hostile (or in some cases, uncomfortably far too friendly). And some people don't want to pretend to be someone they're not so they can fit in with the rest of the hyper-masculine community. I just got used to it and it doesn't bother me now, but back when I first started playing, the toxic environment really got to me sometimes.
As for why there are no females in LCS and Challenger:
If the above reasons don't deter them from playing, I think it's the same reason there aren't as many female computer programmers and engineers. Most of my friends think I'm weird and maybe even socially inept for liking this game enough to play it everyday. It's like that for males too, but the stigma goes up higher if you're a female. So many times I see male friends getting together to play League and I envy them, as my own best friends are always trying to get me off that "lame nerd-game." It's hard to put in that much time into it.
As for the 'time of month theory'.....I myself haven't found much difference in win rates due to that. Lol.
If it's true that "girls are not as good" as men in the game, I would think that it's because they're just socially (in and out of the game) discouraged to play this game and thus cannot put as much effort into getting better.
Trust me, there's a lot of women playing League but guys don't pick up on them since most of us don't go around advertising it. Some people of course go full on gamer-grill but that's a minority of the female player base.
As for me I would love to see real female gamers out there in the pro scene. Yes there had been a few in the past, but they were pretty much in the service of male fans and I'm convinced that the requirement was "you should be kinda good at the game but you def need to look hot" which isn't good for anything competitive.