Oh, I've done this, let me tell you why:
1) People in tech are assholes to women. I don't think most of them realize they're doing it. Sending images of swimsuit models over slack or otherwise sexualizing your work environment is an awful idea, and you shouldn't do it even if you don't have women on your team. My partner gets asked out by coworkers regularly, even while pregnant.
2) No really, people in tech are assholes to women. I've heard women give presentations where people talk over them the whole time, and then critique them for not being prepared afterwords. Nah pal, you literally stopped her from presenting information. People do not take suggestions of women as seriously wrt technical problems.
3) Kids. State of paternity leave in the U.S...
4) Being pushed into management. Women are pushed into management at a disproportional rate -- while some see this as a success metric, it can also be seen as an extension of point 2.
5) HR problems. Using HR sucks when you're the only woman on the team. Having someone else call HR sucks when you're the only woman on the team because everyone assumes it was you.
6) Culture, combination of all of the above, lack of a critical mass of women to make actual change, etc.