Because who gets promoted to management is entirely based on merit, right?
Sadly no. In my experience, who gets promoted to management has more to do with who you're friends with than actual ability.
Please note that gender and race were not mentioned *once* in the above.
But they're still there. What you've described constitutes deep and systematic racism and sexism that place serious obstacles in front of anyone who isn't the right race and gender. Just because no one is doing it "on purpose", that everyone has good intentions and thinks they're doing their best to be fair doesn't mean it isn't happening. It's the result of pervasive unconscious biases.
So, how do you overcome those unconscious biases, break the stranglehold of the good old boys' network on management positions (or a thousand other similar structures)? How do you root out the unconscious biases and make the people who hold them see that they do? Remember, these are well-intentioned people who consider themselves to be kind, and fair... but they just tend to hang out with their own kind, so that's who they know, and who gets promoted.
Serious question. What's your answer? Just letting the self-reinforcing system continue isn't a good one. So what do you do?