There may be a little bit of a selection bias going on - as you described - but it is worth remembering that back in the 50s and possibly 60s system administrat

There may be a little bit of a selection bias going on - as you described - but it is worth remembering that back in the 50s and possibly 60s system administrators did used to be quite a female dominated role because they used to manage the cypher machines during the second world war while the men went off to fight and thus that trend of seeing women in server rooms continued a little while after. Sadly the designing and building those machines were still male dominated - the female role was more administrative than what we now think of as a "sysadmin". As for what caused the shift to become so male dominated. I couldn't say for sure. I have a few theories but I wont share them as they're largely speculative.