Here are my thoughts on why:
1 - marketing of magic is geared towards males
2 - no famous female magician role models
3 - in the media females are still often portrayed as the magician's assistant and not the magician
4 - perhaps, for some genetic reason, women tend to be less interested in performing magic, but I don't know what reason that could be.
What's funny is, I think a lot of women are interested in seeing magic being performed, so it is puzzling as to why more women wouldn't get into performing it.
If I think of my wife though, I know she would never be interested in performing magic, and from what I have heard from a lot of magicians, their wives are the same way. Makes it hard for us to justify certain magic purchases to our wives
