girls can have very psycological ties to their vag's. If they had a bad experience they can shy away from typical sex because of its "intimacy" factor. They are litterally letting a guy inside them. The anus however is associated with garbage and dirtiniess. It's much easier to seperate yourself from something you consider to be a source of waste.
Chances are these girls turned to anal to satisfy their need for sexual exploration/satisfying their mate without really letting them in. I'm going to assume its a defensice mechanism on their part, that allows them to seperate their emotions from the physical act of satisfying their partners. (though I'm sure they would deny it claming regular sex "hurts" or "doesn't feel good" or other variations of "i'm not comfortable with it". which further reinforces this concept.)
In most cases I would assume they were either abused sexually or raped in their history. It would probably be an up hill battle to get them to open back up to it because it would mean facing their issue.