It seems that most Sisters who don't really know what to do join the Red Ajah. They've had the most specific and important function in the Tower for three millenia, and yet they very rarely get to do it. I wouldn't be surprised if most Reds had never even encountered a male channeler.

The other ajahs have focussed functions that usually demand extreme dedication i.e. acquisition of knowledge, diplomacy, Healing etc. Many women might not find joy in any of those fields, at least not to the degree to want to do it all day, and I think it's no small clue that the Reds are the biggest ajah.

Half the time we see Reds they're teaching novices or Accepted, or being sent off as ambassadors. It's a good fall-back ajah for women who have general usefulness but not great aptitude for the other ajahs. I suspect the Greens could also be a fall back ajah if a woman finds the idea of being Red intolerable.