All movie roles aren’t created equal, and sometimes one doesn’t actually work out right until you make some major changes — like subbing in a woman, for example.
When it comes to sci-fi and action, movie studios typically feel more comfortable with a male actor in the lead. Call it a self-fulfilling prophecy, but male-led films typically do better (probably because there are way more of them to sample data from, but we digress), so studios often skew in that direction when they’re making casting decisions.
But, thankfully, it doesn’t always work out that way. On occasion, a male role will be tweaked to allow for an ass-kicking lady to step in and take over, and those little changes have accounted for at least one great sci-fi film.