Even in Europe, there were women who covered their hair. In fact, when someone from one the families of one of the Roshei Yeshiva of Telz was a kallah and she was still in Europe and her chosson was in America, he wrote her a letter in which he mentioned that she should make sure to cover her hair when she gets married. She was very offended and wrote to him asking him how he could possibly think that someone from her family would need such a reminder. He responded that she has no idea what things are like in America and there really was a reason for his concern.
The point is that even in Europe, there were always people who were aware that women had to cover their hair, and the wives of the Gedolim and Roshei Yeshivas did cover their hair. And the Rabbanim did tell women to cover their hair. In the US today, the daughters of the Gedolim do not braid their hair and the Rabbanim are not telling people to braid their hair.
Also, in Europe, most girls were not educated and did not know how to learn, so they had no way of knowing what the Shulchan Aruch and Mishna Brurah say (if the Mishna Berurah was even written yet).