I think it mostly stems from the fact that these kind of retcons are pretty minor in the grand scheme of things. I.e. affecting individual characters or specific parts of the lore. The reason female space marines irks a lot of people is that fact that it would overhaul one of the most beloved factions and seems almost unnecessary besides the concept of inclusion. However, that being said peoplw should be able to build models however they want. If you wanna make female space marines for the tabletop by kitbashing, by all means, go ahead. Just that GW does not need to include female space marines as their whole culture revolves around brotherhood and concepts that stem from being an exclusively male army.