The answer is pretty simple: Lack there of female players at the highest points of the ladder.
Keep in mind that (as far as I remember) the male-to-female ratio players in league is like nine to one.
Then consider that the Challenger portion of the ladder is like the 0.02% of the total player base, and simply by odds, it's gonna be REALLY HARD to find a girl playing at the highest level of the ladder.
There's stories of male super stars of Solo Queue brought to LCS/Challenger teams who choke just because how hard and different it is to play at that level.
So... when you put everything together, you'll see the odds for a girl to pop-up in a pro team are really slim. Remilia stepped down from Renegades for personal reasons and because she wanted the team to succeed and actually felt they would do better with another support. Then there's Yoonie (at the top of my head) and she's currently Diamond 4, but D4 is nowhere near the level you need to play on the pro level.
So, as many have pointed out, it's not a matter of sexism, but simply teams looking for the best players they can find.
If suddenly there was this AMAZING gal playing... dunno, Top. And she had a respectable champion pool and was in the Top 10 in the Challenger ladder, I'm sure someone would probably pick her up. One of Remilia's problems was her champion pool was really shallow. Like... "Thresh and Morgana only" shallow.
So, could it happen? For sure. But the odds of it happening are SUPER slim, at least for now, simply because of how the population of the game is.