This is pretty interesting to me too, I'll chuck an idea in; honour amongst thieves. I've hung out with a lot of dodgy types and a consistent principle is that it's wrong to antagonise a woman - obviously this is usually borne of the idea that women are weaker than men, but let's generalise here. If you rob a dude, it's not gonna be pleasant but you get your stuff and he fucks off. Rob a woman and she will probably cry, and even if you're a give-no-fucks sorta guy, making a woman cry doesn't feel very good.

There's also the fact that bystanders are more likely to help a woman or heed her calls for help, and a woman would be more likely to run away/call out than a man who may be more inclined to stick to the 'honour code'.

Of course I'm guessing here too, I just think that's the reason really. Recently a mugging near my house (three people with one machete vs. seven students outside populated houses... idiots) didn't work out because one of the girls panicked and ran away, the thieves fled worrying she'd call the police. I don't think they'll be targeting any women in the near future.