There was a Mad Men scene (S5E01 I think) where the firm advertises that they're equal opportunity, and when a bunch of black applicants queue up, the hiring manager says to them (paraphrasing), "Yes, you heard right, we're equal opportunity and do hire blacks. But our only openings are for secretaries, so the men can leave."
That is, he's progressive enough to hire non-whites, but still doesn't think men can be secretaries. (In fairness, they probably wouldn't have wanted the role anyway.)
Lots of reasons, but a major one is that until relatively recently that's one of the jobs they were permitted to have: http://scholarship.law.cornell.edu/cgi/viewcontent.cgi?artic.... In the legal world, for example, law firms openly rejected top applicants solely because they were women well into the 1970s.
People tend to think of that time as ancient history, but it's not. C and UNIX were developed in the late 1960s and are still the bedrock of the Internet. Why should we expect social phenomena would move faster than computing technologies?