I think the very fact that there is something distinct and different about an all female group might be part of the reason . Good musicians don't want to be in a band which so many would still regard as something of a novelty act .

The problem doesn't arise for men . Nobody would turn around to a man and say , "You're in an all male band ? wow that's cool/unusual etc" Women who just want to play and win recognition for the quality of their music might feel they have to go into mixed bands to stop being bracketed in a non-musical way .

What stands out about the Dixie Chicks, for instance ?Why should somebody like myself who has never actually heard the group's music , know about their existence? Eight years ago I would have probably registered that an American band took a brave stand against the Iraq invasion ,but I would have certainly forgotten their names by now if it hadn't been for the fact that they are an all-woman band.