Some good points raised. Perhaps most female comics aren't cut out for the aggressive, one liner, bearpit atmosphere of the panel show genre and there's nothing wrong with that.
More likely, the female comedians coming through into the television world are chosen because they are safe, bland and inoffensive. In a genre that is becoming increasingly female led in terms of production and commissioning, there really can't be any excuse for sexism.
Holly Walsh I think typifies this type of new female comic. She's not particularly funny or interesting, but neither is she outrageous or controversial, she just seems to be there, doing not a lot. (except waiting for the cringeworthy pity laugh from the audience presumably)
But as others have pointed out, if you are making a panel show, then you are going to choose safe hands to helm the ship. Nobody wants to bin an entire episode of a series in the name of affirmative action.
Maybe they should do an all woman panel show - not that I'd watch it - but it might be good for morale. They could even have a sing-song and a bit of a dance at the end like those all woman sketch shows do.
'How about a greasy pork sandwich served in a dirty ashtray?'