I think it just is the case that there are more men in IT.
Yes. There's around 23% - 26% women on average (depends on which stats you're looking at.) My employer is about average - but it's balanced by HR being around 51% women, and my own techy division (last time I had figures, not sure how it's changed in the last couple of years) was about 8% women.
According to one of the Computer Weekly/Mortimer Spinks Women in Technology reports (can't remember which year off the top of my head), 67% of women in IT have experienced sexism at work. Sometimes it's really bad harassment, but often it's just on-going daily microagressions, just things like signs on datacentre doors warning about "men working behind doors!" as if there's no hazard when it's a woman in there lifting floor tiles. Or yet another email addressed to, "Gents," even though I'm included on the distro list. None of these things are a big deal as a single event, but they add up so you're constantly getting messages that you don't really belong here in our techy environment.
And then there are things like maternity leave - some areas of technology move on quickly, and if you take a year out, your skills will be outdated. Most companies don't really have much in place to help women back into the workplace and get training on the new things which they need to know. And there aren't that many examples - you get to hear about people like Sheryl Sandberg, but in my direct reporting line, I'm the only woman for the next 5 layers till you get to an American woman, and then there are a couple of more layers of men to the CEO. This experience isn't unusual.
There's an awful lot of generally crap management in IT - this isn't so much a sexist thing, it's just that in many places, the career structure is that you can go so far up a technical ladder - and then you either sit there forever, or you have to go into people management, regardless of whether you have much in the way of people skills. I suspect this isn't unique to IT either, but on top of all the other shit women may have to deal with, you just get to the point where you think, why bother?
Also, I think that popular images of STEM don't help - programmes like the IT Crowd and Big Bang Theory. Many people aren't aware of the sheer range of roles in IT these days. It's not all coding and hardware (although there's nothing wrong with those, either.)
And I know this isn't really on topic, and I could go on for a lot, lot longer (and if you search my posting history, you'll find I sometimes have done), but it's a subject close to my heart. I am quite involved with promoting IT careers, especially to girls, but more and more I'm thinking, I'm not really doing them any favours, encouraging them to work in this industry. On the other hand, if we don't get more women in, it won't improve.
Anyway... as you were. 