Ask HN: Why would someone look for a female CEO?

I just got an email from someone looking for a female CEO. I'm not female, so I don't understand why I got contacted in the first place, but it made me really curious:

What is the reason that someone would specifically be looking for a female CEO? Is this normal in the US? Isn't this the same as someone looking for an [insert race here] CEO? It this even legal?

I'm sorry if this questions comes off as offensive or naive, I know it's a sensitive topic in the US, but I truly don't understand.