First of all, congrats to Ms. Mizrakhani for her award, and it is indeed notable. That put aside, there are a few important reasons why I think Maths education is f***ed up in university which prevents more girls and women from doing it. These reasons are:
While learning maths, the tests are given without an open material, and often require memorising proofs of many pages. This is while a good mathematician can easily look these up and does not need to keep them in his resident mind and that a mathematician or other real scientist is more about deducting and inventive thought than about memorising.
Finishing a maths degree requires a complete buyout into the system, which risks transforming the students into Captain Nemos who are cynical, destructive, people who think they are a "nobody", which is what "Nemo" means in Latin. Also see what I wrote about it in a different context.
Now girls are by their nature, have been more unwilling to become Captain Nemos, and also realise that in this day and age, being an amateur, who are people who love (= 'aime') what they do, and/or who cut corners and disobey the rules, or are willing to produce somewhat less stellar results, is much better than being a professional, which is a mostly 20th century fad. It is well known that in many fields of endeavour some people who are underage, and/or inexperienced, and/or less professional can beat the pros at their own game: software development, music, acting and film making, martial arts and other combat fighting, modelling, writing (blogging, novelling, etc.), being a waiter/waitress/shop clerk/shop vendor/etc, cooking, even sports. And yet maths education in our f***ked-up university system believes that a mathematician should be a "Captain Nemo"-like professional than a happy, well-rounded, polymath, amateur (a.k.a a "geek").
You can also see what I wrote about amateurs and hackers (a.k.a "Action heroes" in a different ccontext.).
Another problem is the fallout from Euclid's reported “There is no Royal road to Geometry” adage. Thing is, when teaching maths, you can and should skip some stuff and show the cool stuff. There is no need to teach the very basics and instead one can skip stuff. I recall that we didn't learn the Jewish Bible from its beginning to its end, and we also skipped eras when studying history, and stuff like that. A lot of the material I had to learn in my Electrical Engineering degree, such as the physics of semiconductors proved of zero utility to my work as a software developer, and later on as a writer/entertainer/philosopher, which is what I am now.
There are other problems with the academic world: instead of collecting donations at the end of the lectures or otherwise getting a motivation to be popular (like philosophers did at ancient times), the so-called scientists/philosophers are getting tenured positions, and don't want or need to try to improve (which makes their students unhappy). Currently, the world's best philosophers (or in their modern name: "scientists") are the various entertainers of the world: actors, screenwriters, authors, bloggers, models, musicians, T.V. celebrities, YouTube/etc. artists, talk show hosts, etc. etc. 50 or even 20 years from now, people are more likely to remember a famous actor, directory, blogger, or even - model - than a university professor of philosophy, which I cannot name a single one, and do not care to remember any one of them. And many more people are likely to read or watch an interview with Jennifer Lawrence, Emma Watson, Candica Swanepoel, or even - Jimmy "Jimbo" Wales etc. than they are with an obscure, and likely boring, contemporary professor of philosophy.
This does not have to be this way.