It's your classic bootleggers-and-baptists coalition - the term coined by public choice theorists to explain the strange alliances which tend to get things done in liberal democracies. Bootleggers and baptists both loved prohibition, for entirely different reasons, and together they were a powerful force behind laws restricting the selling of alcohol. The combination of high- and low-minded interests is often a winner, and it has done a number on SF and fantasy; some people care about diversity, some people have spotted bucketloads of cash going unclaimed - either way, there sure are a lot of Strong Female Characters around these days.
Not that I have a problem with Strong Female Characters. But I do have problems with certain elements of the phenomenon.
The first is just my objectionable, contrarian, bloody-minded nature: I hate being preached at, and sometimes there is an element of preachiness in an author or film-maker electing to have a Strong Female Character in a certain role - a certain sense of somebody waggling his or her finger at the audience and saying "Now, don't you be a naughty sexist and think there's anything wrong with a woman being in this role, and by the way, didn't you know women could also be engineers/soldiers/rugby players/mighty wizards/whatever?" When I sense this motive I instinctively recoil, like a slug being sprinkled with salt. Nothing makes me want to be a sexist more than a thinly-veiled lecture on the evils of sexism masquerading as a character.
The second is more serious: I understand that nowadays you're not allowed to say this in some circles, but I'm one of those befuddled lunatics who thinks he has observed that men and women tend to differ in certain important respects (on average, always with exceptions) and that being a Strong Female Character probably ought not to just mean a Strong Male Character But With Breasts. I have known, liked, loved plenty of strong women in my life, and I've never thought of them as strong because they are just like men. I know some great women karateka and judoka who could break your arm as soon as look at you. I know some women who have risen to the pinnacles of their professions. I know some women who can lift heavier weights than I can. That's not what makes them strong, because (here's a life spoiler alert!) that's not what makes anybody strong. Strength of character is what counts. And male strength of character tends to be different than female strength of character. When I see a Strong Female Character in a book or on screen, I want to see a character who makes sense to me as a woman in view of that.
To me the paradigm example of somebody writing a Strong Female Character well is this scene from Thomas Harris's The Silence of the Lambs (which is the last good book he wrote, by the way). Clarice has been sent to look at the corpse of a brutally-mutilated female murder victim in a rural mortuary. She arrives to find that the place is crowded with men - local police officers - who have been joking around with each other. She's intimidated but it's also implied she's affronted by the lack of dignity in what she sees. Here's Harris:
"Starling took off her scarf and tied it over her hair like a mountain midwife. She took a pair of surgical gloves out of her kit. When she opened her mouth for the first time in Potter, her voice had more than its normal twang and the force of it brought Crawford to the door to listen. 'Gentlemen. Gentlemen! You officers and gentlemen! Listen here a minute. Please. Now let me take care of her.' She held her hands before their faces as she pulled on the gloves. 'There's things we need to do for her. You brought her this far, and I know her folks would thank you if they could. Now please go on out and let me take care of her.'"
They suddenly become quiet and respectful and file out to leave her with the corpse, and she makes significant discoveries as a result. Apart from its understatedness, what impresses me about this scene is that Harris doesn't adopt the line a lesser writer would have taken, the preachy line, the line of "Well Clarice Starling is a tough cop and so she butts heads with the men and shows them she's in charge." He takes the realistic line: "Clarice Starling is a tough cop and she knows how to empathise with people and how to say the right thing, the thing which will get through to them and get them to do the right thing." He gets that men relate to women differently than they do to other men, and vice versa. It's kind of immaterial that this might be socially constructed, as the inevitable response will be. The point is, whether it's socially constructed or not, it is how people actually are.
(Thomas Harris is a weird case study in the fame of writers. There is a chasm between The Silence of the Lambs and Hannibal. One one side, there is a powerful craftsman at the height of his powers. On the other, there is a madman who has had a rush of blood to the head because so much critical praise has been heaped on him that he's lost all connection with reality.)
Such characters are, regrettably, thin on the ground in SF and Fantasy. It is a cliche to point to Ripley in the Alien films but it's a cliche for a reason: Ripley is the classic example of a female lead who you couldn't replace with a man just as easily. Yes, she can fight, yes, she is technically accomplished, but the themes of maternity, compassion and empathy are what gives those films emotional depth. A male character in that role just wouldn't bring half as much to the party as Ripley does.
Another one is Princess Leia, who participates in the action just as effectively as the men, but who also provides an emotional core to the Star Wars 'gang', bringing some sensitivity and nuance into what without her would be a somewhat by-the-numbers boys'-own adventure story. (Think of the scene between her and Luke in Return of the Jedi when he tells her they're brother and sister - Carrie Fisher's acting is underrated; given a chance to show she had a range she was perfectly good at reciprocating.) It's not that the men get to have all the fun and derring-do and she's at home to patch up the bruises and cook them dinner. She's perfectly well involved. But she adds depth that another male character wouldn't.
Finally, I really like the Cordelia character in Lois McMaster Bujold's early books - somebody hard-headed, practical, who gets to deal with genuine ethical dilemmas (how often do Strong Female Characters get to do that in Hollywood films?) but who also leavens the violence, hatred and militarism in the male characters in the book with forgiveness and empathy. (I really ought to re-read those - it's been a good long while.)
I close in hoping this post doesn't engender (pun intended) a debate about "political correctness gone mad", "feminazis" or "mansplaining" - take the post in good faith and give some good examples of Strong Female Characters of your own.