"more chauvinistic and sexist industries (eg: finance, advertising, previously medicine) appear to be having little trouble attracting women"
I can't speak to advertising, but finance and medicine are some of the few employment traditionally open to women: tellers and nurses. Of course, they aren't traditionally represented in the higher levels of those industries. That seems to be changing, but I suspect the "traditional field" aspect is the key-in-the-door there.
Software, and similar science, technology, and engineering industries, don't have those kinds of traditional positions, making the imbalance more obvious and less, well, fixable.