Mangoose
Arcane
Yeah.Well there's that, but in universe it still doesn't make sense.
Space Marines in game are the most common faction, but in universe there's very few of them and they are supposed to be incredibly rare and precious, but since GW wants to keep selling marines they get put everywhere and are displayed in huge land battles that per their own fluff would them all killed.
Modern 40k fluff isn't great. It wasn't flawless, but ever since 5th ed it's just been getting worse and it doesn't help that GW is trying to sanitize the setting, turning their grimdark universe into yet another marvel wannabe with bright colours and silly model designs.
And it was all because of fucking Ward and his Ultramarine and Grey Knight fetish.
Honestly, Black Library should be considered almost a separate element from the rest of Games Workshop.
I mean, they're even different in terms of lore, as you know the codices provide lore too... and Matt Ward wrote that game-side lore.
Well that and the $$$.. and they don't really care to make the main 40k game as a real thinking strategy game (intentionally so, too, as usually the ppl playing 40k at gaming shops are LARPing like crazy).
It's the off-shoots like Kill Team and Blood Bowl and.. even Warhammer Fantasy.. that focus on the gameplay. (In fact my favorite wargame I've played is Infinity)
Fake-edit: Oh I also forgot about video games, where there is a decent number of good 40k adaptations that don't just try to follow 40k mechanics too too much. I mean, the main reason 40k got popular was Dawn of War.
Real-edit: I actually got interested in 40k around 5th edition. So, yes, I know so much about the Ward effect. Which is why it's so surprising Guilliman is good now. I mean, even though partly it's because of Dan Abnett's "No Know Fear" but I didn't expect anyone to be able to conquer what Ward did to the smurfs.