I think it's all the more remarkable that Lupino did it coming from the factory environment and with the success of being at least a second-tier leading lady. But the fragility that the industry encountered in the high-water period of noir, what with the Blacklist, the Paramount case, and the rush to the suburbs, pushed a lot of people out of their studio associations, and the moment came for her to go independent. It's possible that had she not done it in '48-'49, she might not have been able to make the films she did, even though the deal she and her soon-to-be ex-husband Collier Young made with RKO was in no way financially advantageous.
And that brings up the fact that it was Collier Young who found a way to thread the needle in the post-war studio environment that gave Lupino her chance to direct. Women apparently still mostly require male mentors or facilitators to get in the door that way, which is a kind of continuing scandal in present-day Tinseltown. (Maureen Dowd just wrote a cover story for NYTM about that which ran this past Sunday.)