Part of it is a focus on the word "Space Marine." Space Marine is, within 40k, a technical term to describe a very specific group fo people, whereas the word "Space Marine" as it is actually used in the Sci-Fi world has a more generic usage. i.e. it's a name, not just "the people who fulfill the role of marines, but in SPACE; and also COOLER." or "people with armor! And guns! And inexplicable melee weapons! IN SPACE." There are most definitely women who fit the latter definitions in 40k.
Space Marines have always been all men. I don't remember if there was ever an in-universe reason for it although I'd bet there is. Suddenly having female marines and pretending that it's always been that way would be kinda dissonant. It'd be like suddenly giving Gordon Freeman full voice-acting; not directly contradicting anything, but against pretty long-established rules. The alternative would be advancing the universe of 40k in order to introduce a reason for female marines to exist, and I understand GW would rather self-destruct than advance the plot of 40k.
Either retcon or new canon, there's a problem: because it's about gender, it will feel contrived at this stage of the game. It'll feel like a choice made so that the game could stay relevant, rather than because whoever writes 40k lore genuinely decided to add it. I wouldn't call it pandering, but it feels artificial and game-y. Like breaking the 4th wall on some weird meta level.
I don't give a damn about the lore of 40k, I just like the memes and videogames. I think female Space Marines would be cool too, all of what Dunamisdeos says applies. At the same time though I can understand why it would rub /tg/ grognards who've been playing tabletop 40k since the 80's the wrong way. I have no idea if what I said above is what they're actually saying, I just tried to put myself in their shoes.
Now see, this is a reasonable approach. Were you building 40k from the groundup, you probably wouldn't design it this way at all, but GW is not doing that (and can we say thank the Emperor for that! Never forget what happened to Fantasy...).
Also, anyone who thinks GW won't move the plot forward... well, they blew up cadia so I think they took a thunder hammer to the status quo
There's still hope. It's not moving the plot forward until it's the 42nd Millenium. Until then it's just a Zeno's Paradox countdown to Midnight/Doomsday, where it keeps going but never arrives anywhere.
But anyway. They have the established conventions, the established fanbase, the established a lot of things; of course GW likes to mess things up, but it's one of the only things they've even been totally consistent on. You'd have to rock a lot of boats to get it added, you'd hear an
endless stream of bitching (if not worse) from the people who give the company their most reliable dollar (and 4chan would raise an unholy tantrum), and the complaint that it's forcing in diversity is, not wrong? I mean from a story perspective, the Imperium is quite well established as incredibly, mindbogglingly conservative about basically everything (which is the whole point of 40k! The Imperium prides itself on being as closeminded as possible at all times), so it's not like they don't have grounds for saying it's discordant.
Not a lore one so much as a symbolic one. They're the emperors prodigy in a way, made from the closest things to sons he had and they continue that legacy as the chosen of mankind. He chose to make them all men and until he wakes up to say that doing otherwise is allowed I'm pretty sure it's considered heresy. It's why women were given a chance to serve trough the SoB branch (and I guess the IG, but those aren't modeled afaik) which are about as close to proper SM as they can get.
For my bit, I'd say that the fact that SoB and female Imperial Guard are under-represented
despite being very lore-friendly is much more of an issue and a sign of some sexism (in the real world, not 40k) than the space marine thing. I don't play Warhammer myself, but from what I've heard it's a hard life being a fan of the Sisters of Battle. People focus on the space marines because GW obviously favor them and they are the face of the franchise (and the nerd fantasy of the kickbutt action guy who doesn't afraid of anything), but I feel like the focus ignores the underrepresentation elsewhere.
From where I understood it, alongside all that was said and I agree with, visibility may be an issue too
I'm of the belief that many people got their start in knowing 40k by the games, and in said games there's a lot of visibility for Space Marines (and their many colorful chapters), and given where players start off -- their basis can come straight from IRL situations. The Sororitas only seemingly have one Order being represented (The White Rose), and may give off the impression that their hairstyle is...only white (which is characteristic of that Order but not of the other orders in existence), and...I'm unsure about the comparison between how available tabletop gaming is (I seriously wish we had it HERE IN THE PHILIPPINES [or D&D :V but I never got to play any]), but it seems more that the visibility is through the games first. That and there's that note about the Emperor and female space marines before--there's no discrimination anyway, and more focus seems to be placed on the SM's which may cause this kind of thought-out.
Didn't matter to me in the first place though, as it made sense in lore, and that if I was a female aspired by the Space Marines (eg My world is under attack D: I want to serve the Emperor! Space Marines saved me people via drop pods!), it's not like I have a ton of other choices that are just as cool or inspirational despite the ton of darkness and grim feelings every bit of them may provide. That and the most popular SMs aren't jerks, even if there are jerks everywhere And while the Ecclesiarchy can be between 'extreme' fervor and balance, there's still a nice spot in there to consider the Sisters.
That, and reading up the lore, that there's a lot more IG (Female or Male) being re/presented in helping folks and saving worlds than Space Marines given the number galaxy-wide. So...that's what caused me to say visibility.
That and I read a nice game on spacebattles(?).com about a neutral chapter of Space Marines in some quadrant of the galaxy with its head being a female sorceror who isn't really bad or with Chaos, but has been forced into her position by Chaos (or whatever) but stuck with the conundrum that seemingly is visible in the 40k universe: What choices are present when 'this faction' controls 'your side of space'.
Or Tiruin could come in and steal my thunder.
Also, I just briefly hallucinated (it's like a sudden flash, or lapse rather, where your eyes connect things in a way that they aren't supposed to connect) that words of this reply page formed a very distinct image not unlike an Ork, or monstrous hippopotamus. Maybe I should get some sleep.