I know exactly why it's happening nowadays. It happens across many narratives, and the sad thing (or maybe it's the scary thing) is that it's actually being done by those supposedly championing the specific cause. The easiest way to envision it is to use a racial example, but it can easily carry over to the gender, sexuality, and other issues, too.
If a black person becomes successful, they are almost always written out of the narrative of history, and if it is happening right now, they are discredited. Successful African Americans like Dr. Ben Carson, or Herman Cain are called Uncle Tom's or worse by leaders of the moment when they offer advice on opinions on how to become a success in current society. When you think about it, they've even done it with Martin Luther King Jr. We always hear about his greatness in "wanting" equal rights via his "I have a dream..." speech. Have we ever been taught about the things he actually ACCOMPLISHED in the equal rights movement? I imagine many would be hard pressed to come up with anything. (Here's a few for those playing along at home: Forced the Supreme court to rule that segregation on buses was unconstitutional, was instrumental in ensuring that the Civil Rights Act of 1964 got passed, and many more).
The reason for this phenomenon is simple, in order for a some sort of cause like civil rights, women's rights, or anything to maintain momentum and continue growing, there needs to be a sense of urgency. The Global Warming/Climate Change movement would never be where it is right now if there wasn't the "We need to move now or the world will come to an end!!!" angle. In order to do this, we need to ignore (or hide) all of the amazing and incredible advances we've already made since the late 1960's when the movement began. So, to keep things going, the Green Movement needs to focus on the doom and gloom and glaze over all the advances in sustainability, recycling, auto emissions reduction, and so on.
The same rules apply here. Leaders in the race movement need to devalue the accomplishments in order to increase the value of continued work in the field. Leaders in the feminist movement need to devalue women's accomplishments of the past in order to increase the value of pressing forward even more. If you teach the women of today about how large a role women played in early tech (nearly 40% according Ms. Smith's statements above), then it's harder to convince them that it's important to fight for their rights, now.
It's sort of a Catch-22 really. Things are getting better in most of these areas (and have been for a generation or two) but to acknowledge that is to run the risk of slowing the progress to still be made (because something else will surely turn up that is more unjust or scary and take away its steam).
It's worth noting that this is starting to happen in the gay rights movement, too. It doesn't quite show up as much right now since we're a lot less further down the road in that area as we are in race, gender, and ecology issues, but it's there if you look for it.
Ultimately, I'm not sure which is worse - rewriting (or at least hiding) history, or running the risk of letting a movement fall to the wayside because complacency has been reached. The idealistic side of me wants to believe that once true equity is reached, then we can go back and unrewrite the history, but the realist in me knows that probably won't happen.
I guess we need to look at each case and ask ourselves, do the ends really justify the means? The trick is that there probably isn't an absolute "right" answer to that question, but at least we can understand why it is happening. Understanding motive is important, because even if we don't agree with what is happening, it we can empathize with the motives of those who are making it happen, it's easier not to cast quick judgments.