This looks like the right place to repost this interesting theory.
I heard this from a guy who had lots of stories and ideas. The stories were always reliable; he had the exact famous players and circumstances. His ideas were less so.
In Europe long ago, when our people were spread over a wide area, davening with a minyan was a rare event for many Jews. When a boy was born, the father worked to get to a mohel as soon as possible, and the baby was named. On the other hand, when a girl was born, there was no need to hurry; the father would wait until the next yom tov, sometimes waiting until the next Yom Kupur, when he would be going to a minyan anyway. And then he would name his daughter. But what do you call your baby in the meantime? Zissel, Shaindel, Feigel, Gittel, Sheina, etc.And that's why there are so many Yiddish names for girls and relatively few for boys.