I don't think there is a specific reason, and I don't think it's patriarchal, it just so happens to be so far.
But if you want a logical reason, in reseaech studies, women are typically much more task oriented an organized than men. They're also better at object placement memory. Not to say men can't do it either, but it's hypothesized that it's an evolutionary bias.
You can follow on the WT wiki for more clarification on it.
This is base of my memory but I’m pretty sure there a QNA section on that.
Ashihara say there are male operators in the verse but they aren’t really on duty. And why there a lack of male operators, in the WT world, operators and engineers path are usually recommended to those who can’t join/continue as a border combatant, and it just more male would perfer to go into engineering rather head to the operator school. And most if not all Operators we seen, they are pretty much top of the line when it comes to their operator skills. I didn’t see any patriarchy smell as it just really show how talented these female operators are in the universe

always wondered this but is there a specific reason all the operators are all women or it's just a thing?
I was rewatching the show and noticed