why aren't there more successful female comedians?

as i write these words, sarah silverman is appearing live in london. the broadsheets have printed many words about how such a cutie can have such a potty-mouth. it is a question that interests me - what are the qualities of a woman that is considered to be funny. the above promo for a filmfest that happened earlier this year contains some sound-byte opinions of women well-known in britain for their work.