it's 100% because of the gaslighting.

tell me it wouldn't be the most 40k/Metal thing to just let women throw themselves at the training with zero prejudice and the 0.000001% that survive are just so much better than the men because the odds where so much worse. You could even hard retcon that in and no one would be mad because it's plausible that it's just never happened before so of course it's never been in the lore.

it's not like GW are bad at this sort of thing either, they got people to accept Primaris marines and that's a much bigger load of horseshit from a lore perspective.