My answer is that women were never there to begin with and the number of women has always been fairly constant.

My mother started her degree in the 70's because CS was a relatively new field and seemed interesting (let's not forget the "computers will solve the world's problems" marketing of the time). She took lots of programming courses and discovered that she was good at programming (and sometimes did a bit of coding at work), but preferred administering the machines and doing keypunching (and later other forms of data entry) where programming is minimal. From what she has told me, this was fairly common for the women she knew (not a matter of sexism nor of ability -- only of preference).

As the 80s progressed, computer jobs shifted. Data entry no longer required a CS degree, so although the number of women doing that job increased, they were relegated to other non-CS degrees (and weren't really interested in CS to start with).

As a more general idea, I think former Harvard president Larry Summers was potentially correct when he said that studies have repeatedly shown that men and women are different in many ways and some of these (IQ for example) have a huge impact on STEM (he was then forced to resign -- not because anyone disputed the numerous studies in this area, but because it is politically incorrect).

For example, men outnumber women in the top and bottom of IQ spectrum (a couple standard deviations either way) note: the AVERAGE is very similar. There are good evolutionary reasons for this. Dumb women were a greater liability to children than dumb men and the tradeoff of some really smart visionaries (and dumb muscle if you would) worked well for human advancement (but was only possible in men).

As a quick aside, though IQ does not accurately reflect overall intelligence, it does reflect many of the most important the parts of intelligence needed in STEM fields.

At the top of the IQ spectrum (the place where the most successful STEM people tend to be grouped), men outnumber women 2-4 to 1 (depending on the study). The total number of STEM jobs in the USA is 4-5% of the population. If most of those people are in the highest IQ quartile, then we would expect men to greatly outnumber women with women accounting for 20-30% of the people "smart enough" to perform the job. This is very closely reflected by reality and also explains why programs to "create more STEM talent" are largely doomed to fail because you can't train people to be more intelligent. Programs to recruit the best and brightest in impoverished and under-educated communities (where IQ scores represent lack of education instead of lack of intelligence) would reap results, but those don't seem to attract the attention of the powers that be.

Another point of interest is that a higher Asperger's quotient (AQ) seems to correlate to programmers in particular (with most programmers being much higher on the autistic spectrum than the average person). Similar to high IQ ranges, Autism spectrum disorders (ASD) favor men 4 to 1 over women.

A final note about sexism: Sexism exists in every job (and goes both directions to one degree or another). The degree to which it exists in CS is dependent on several interesting factors (for example, that AQ factor which means that it may just be bad social skills rather than actual sexism), but does not necessarily correlate to a significant difference in women and technology (that is, if differences such as IQ are a limiting factor, removing all sexism against women would still result in more men in STEM).

This is true before we even account for women simply making the life choice to NOT be stuck in a tiny cubicle working excessive hours and weekends due to a salaried job while dealing with a pointy-haired boss who has the personality of an agitated wolverine. My mother chose to step out of the rat race and spend time with her children as we grew up (and my father picked up the slack -- despite it's effect on our relationship). When we were gone and she decided to go back to work, she needed to go back to college again because CS changes a lot in a very short time. I suspect that many other women face similar choices and make similar decisions (or simply opt to never try because those intelligent enough to do the job are more likely to make that life choice before entering college).