Maybe Bill D's post provides a clue -- might there be a perception that there are more female ballad singers because they sing more and different ballads? Although we would be no nearer to knowing why they would want to! (Mind you, I've a memory from another thread that Jim said that Ewan MacColl sang over 100 ballads, but he was a very special case and should probably be excluded from any debate.)
Jim, thanks for your detailed answer; there's something lurking at the back of my mind, but I may need to sleep on it.
And thanks to everyone else for being so good as to engage with this question - it's giving me plenty of food for thought!