The fact that there did used to be a lot more overt injustice is making a lot of people angry and resentful of any suggesting that there still might be a problem. This observation explains about 75% of Slashdot posts on the subject.
Once women were treated really badly. Beating your wife was considered a reasonable way to discipline her, and in fact it was your duty as a husband to keep her under control. Many areas of life were simply off-limits to women. It was really bad. So women campaigned for equality and liberation, and got a lot of it. Most modern consider people who beat their wives to be bad people, again rightly so.
The problem is that many men now consider a sexist to be someone who does things like wife beating and overt discrimination. They think of sexual assault as a guy in a balaclava lurking in the shadows. So when someone suggests that there is some sexism or that a particular action might be a form of sexual assault they get really angry. They are not the kind of guy who beats his wife or keeps women down, they would never lurk in bushes waiting to rape someone. Therefore that person must be wrong, because if they were being sexist they would also be doing those things, and they aren't.
It goes further. Someone suggests that video games might be a bit sexist. Some guys reason that because they play and enjoy video games, the accusation is that they are sexist and must be a bad person. That isn't the accusation of course, but it forces them to consider the possibility that something they enjoy might make them a "bad person". It's a ridiculous false dichotomy, but when you look at GamerGate posts it's clear that's what a lot of people seem to think.
So there is a massive push back against efforts to get diversity in tech, because if tech is doing badly then people in tech must be bad people, right? And I'm not a bad person, so the claim that there is a problem must be wrong.