“When did this change you speak of occur? After the establishment of the bais yaakov system? If not the kasha on you still remains as to why this change was not implemented earlier in history or certainly at the latest when the BY was established.”
I would venture to say that there has been a bigger change from 1917 to today then from Chazal to 1917. Look at the life of a girl today especially in terms of general education and interaction with the world. Also, even if 1917 was the same as today (which it wasn’t) starting Bais Yaakov was enough of a chiddush as it was. They may have felt Tafasta Meruba Lo Tafasta.
Yes I am referring to the Divrei Yoel. He quotes the Chofetz Chaim and very severely limits what it would apply to.