I just don’t see the necessity of it considering the overwhelming majority of drivers are already male. This feature is trying to even out the odds of women getting picked up by other women which just isn’t very likely right now.
To seriously answer your question though, this is a marketing tactic to get more business from women who they can see use the app less than men. They’re a business at the end of the day and it’s a way they’re predicting, whether correct or not, to increase sales.