I have nothing to back this up, but my personal opinion is that the reason is our patriarchal society.
It permeates our shared culture to such an extent, that most people don't see it. Women and girls even participate in it, and use it to perpetuate it. It is everywhere. Literally everywhere. From the moment of birth to the moment of death, you are immersed in it. It's not really a wonder that children would pick it up quickly and act accordingly.
If you ever are able to glimpse it - you likely won't be able to unsee it. It becomes blindingly obvious.
You and others may or may not agree with me, so I'll just leave this here:
https://cdn-images-1.medium.com/max/800/1*yiv4glZWr2rzS2BMOo...
Women are less aggressive and assertive by nature.
Testosterone and stupidity (much more prevalent in male population) does wonders when it comes to being aggressive, assertive and risk seeking.
There are social constructs but I would not deny biology. Effect of testosterone and estrogen on behavior has been studied extensively and it is quite clear from that why men and women behave the way they do.
Is women avoiding STEM a social construct? It most definitely is. Is women having lower salaries a social construct? In some cases yes, in some cases it is quite clear that they are not as risk taking as men, and are much more agreeable (they will not risk by asking a higher salary during a job interview).
http://sanfrancisco.cbslocal.com/2015/04/08/reddit-ceo-ellen...
A nice example of how biology created an inequality.
I'm not sure if this phenomenon has been studied in adulthood, but I wouldn't be surprised if some remnant of it remains. Could this simply be an example of that phenomenon in action? Which isn't to say this isn't sexism, I wouldn't consider those two things mutually exclusive.
EDIT: to be clear, I'm not absolving the moderator of blame, or placing any fault on the woman. I'm simply linking this behavior to a broader social phenomenon that is fairly well studied. I personally think what he did was sexist, whether or not that was his intent.