Because the point of the defaults is to give new users a broad starting collection of subs, to overcome the initial hurdle of discovering new subs and to set the tone for the sorts of topics you can delve deeper into. Reddit has a bit of a male bias in terms of targeted content and the userbase per sub, so while stuff that caters to the interests of and is bosted by both males and females gets to the top, there's going to be a natural imbalance on a platform where posted content is curated by the community.

2X isn't "just for females", but for generally female topics that don't really fit into the other defaults. So theoretically it evens out the default experience. I agree that it's not the best solution, the same way you wouldn't address Reddit's imbalance of horticultural topics by making /r/marijuanaenthusiasts a default, but I get the logic.