Why is 50% correct? Why was that right and later lower values wrong? What is the rationale?
"Right" and "wrong" are judgment calls, though most people do feel that keeping women out of a field with no natural barriers to them is wrong. Regardless of your personal values, the historical data shows that whatever barriers exist are not natural.
The fact is that CS -- or what people on HN call CS -- has changed massively in 40 years. For instance, startups barely even existed 40 years ago, and they certainly weren't as hyped as they are now. Furthermore, gender roles have changed in the past 40 years, as has female college admission.
There's something ironically conservative in democrat-styled liberals' [0] harking back to the CS of yesteryear where gender equality was the norm. Societies change, as do fields of work. If I had to take a pundit's guess at the reason, I'd probably say that we have similar raw numbers of women in the field now as then, but more men have flocked to the field as it's proved itself as an excellent means of generating wealth, thus upsetting the ratio.
In all honesty, if I were to guess at the reason there aren't many women in CS, I'd say it's because women don't tend to care as much as men do about generating wealth. Warren Farrell's 'Why Men Earn More' came to similar conclusions: that men are more disposed to taking nastier, more stressful jobs if the pay will be higher. If this is indeed the case, then it would seem that the easiest way to attract women to tech would be to get everyone -- irrespective of gender -- involved in CS at a young age, and allow them to see how fun it can be. If women genuinely do choose to work in careers that they find fulfilling, then this would achieve the goal of increasing the numbers of women in tech without having to discriminate in any way, simply by getting more women to self-select into tech.
[0] I must say, from the point of view of a UK liberal, there doesn't seem to be much liberalism in the US definition of the term. It seems closer to socialism from where I'm standing, with its heaps of top-down reforms and regulations. Yes, this is irrelevant, and no I'm not willing to defend this statement, since -- as a UK citizen -- I have no dog in this race.
Why are so many professional basket ball players black? Racism?
There is an implicit assumption in this and its validity is assumed. It is not being questioned.