It's a comic strip.
No other explanation is necessary, because unless you have some weird mental disorder that prevents you from understanding the concept of fiction, you already know the "explanation" for any aspect of any story that differs from reality: Because the author wrote it that way. Because you are a real person in the real world reading a made-up story in a made-up world, and the real person who made up the made-up story decided to make it up that way for a variety of real world reasons.
Do you need an explanation for why there are dragons when the real world doesn't have dragons? Because it's a story. Do you need an explanation for why those dragons can fly when logically a creature of that size shouldn't be able to do so? Because it's a story. Do you need an explanation for why a human wiggling their fingers and saying certain words causes lightning to shoot out of them and fry that dragon to a crisp? Because it's a story. Do you need a reason for why that finger-wiggling human is a gay woman and not a straight man? No, you don't, because it's the least absurd thing in this paragraph and you accept all of the others without question. But if you do, then it's because it's a story.
Grant Morrison once said in an interview with Rolling Stone:
"Verisimilitude" is a highly overrated concept. If it is not a requirement for the many things that don't exist in the real world and could never exist (dragons, wizards, zombies, time travel), then it certainly has no power to prevent things that do exist in the real world from making an appearance."Kids understand that real crabs don't sing like the ones in The Little Mermaid. But you give an adult fiction, and the adult starts asking really ****ing dumb questions like 'How does Superman fly? How do those eyebeams work? Who pumps the Batmobile's tires?' It's a ****ing made-up story, you idiot! Nobody pumps the tires!"
What's even worse is this idea of statistical verisimilitude that I keep seeing—where the numerical percentage of characters with certain traits must match the "likely" percentages in the real world, based on whatever filter the proponent chooses to determine what is likely. This argument is utter unadulterated garbage. It's garbage because stories are about protagonists and protagonists are usually unusual. There are only like 4 Force-wielders in a galaxy of trillions at the start of Star Wars, yet by the end of second movie they've all appeared as part of the narrative. Is anyone complaining that Star Wars breaks verisimilitude because such a small minority group is so over-represented? No, because it happens to be a story about those people who belong to that group. Likewise, if a story has a percentage of LGBTQ+ characters that is higher than the statistical occurrence of LGBTQ+ people in the real world, does that break verisimilitude? No, because it happens to be a story about those people who belong to that group.
The inherent garbageness of the statistical verisimilitude argument cuts both ways, incidentally. It's not really valid to say, "Half of all the people in the real world are women, therefore half of the characters in your story must be women or it breaks verisimilitude." Not true; it might happen to be a story about people who are male—say, a love story between two gay men. A much, much better argument is, "Half of all the people in the real world are women, and those women buy comic books, too, so you better get your thumb out of your ass and draw some women."
An all-male (or predominantly male) story might be a valid artistic choice but doing so opens one up to an assortment of social and economic pressures in the real world. The artist then needs to decide how much he or she cares about those reactions and what it says about them and their work. It's totally valid to then say, "Screw it, my story doesn't have any women in it, deal with it." However, if one does that, one needs to be prepared to accept the consequences of that decision, which may include low sales, poor critical response, being labeled a sexist (or worse), etc. If one believes in one's artistic vision enough to weather that storm, then hey, have at it.
Me? Not only do I not want to sail into that particular tempest, I wasn't even aware I was on that course until it was pointed out to me. And it's tough to turn a ship as big as OOTS around, especially at this late date in the narrative's journey where there are so few characters left to enter the story, but I'm still trying. It would have been totally valid, artistically, for me to plant my flag and say, "No, OOTS needs to be predominantly male for Reasons," and then it would have been equally valid for people to say, "OK, well, that's not really my cup of tea so I'll go throw my money at some other more diverse comic strip." I have no reason to die on that particular hill, however, as I happen to agree with the general notions of representation that have been raised and am somewhat embarrassed that I didn't notice the problems involved sooner.
When you say (earlier in your post) that you have expectations about gender, what you're really saying is, "I expect a story to cater to my existing worldview," or, to put it another way, you're saying that you want it to be your cup of tea. So if I have some readers whose cup of tea is more diversity and some readers whose cup of tea is less diversity, then how do I decide which type of tea to brew? Easy. I brew my cup of tea and let the chips fall where they may, and that happens to be a narrative with plenty of diversity. If I got the tea recipe wrong before, I guess I'll just have to put a new pot on now.