I can't offer any reason as to why the former appears to be more popular than the latter, I just thought it was interesting and that people might enjoy discussing it! The one obvious patter with male-to-female names is that the majority end in an "ee" sound. Does that mean Davey and Robbie are next?
Maybe because the number of women is greater in the general population and they tend live longer than men, any names that are spread across sexes are more likely to remain (and eventually become exclusive to) female than male. There's probably also some sexist ideology built into the idea that a woman can have a man's name but not vice versa.