Why do women choose not to do tech degrees any more? Those courses used to be more popular in the 70s and 80s. The must be a reason.
Because once the computer became personal computer (instead of a main frame), very socially awkward people (predominantly male) fell in love with them. The women would look around, see handful of class mates absolutely in love with these computer things and then felt out of place because they were not as passionate about computers/programming as these other guys. Since they felt out of place they left to a field where they could feel roughly as passionate about it as their peers.
Other men in the classes wouldn't be doing the "reading between the lines", in comparing themselves to the in-love-with-computers students, and didn't self select themselves out of the classes. Those men just weren't socially sensitive enough, in the way that the women were.