Hmm.....you know, I'm not completely positive, but I have a theory.
I think its not so much that they sell the females young as that they sell the males old. I know most Petstore males are 4-6 months to a year because thats around when their flashy, attractive colors come in making them, obviously, easier to sell. However females, as lovely as they are in their own way, generally aren't has bright and flashy and colorful and don't have the long flowy fins like males......so there is no reason for the breeders/suppliers to wait to send them into the store. They can get rid of the females a lot quicker, so they do.....kind of like the baby bettas really.
Thats just my theory at least. xD