Choppy:
They are furthermore accepting students from different backgrounds to begin with. Most Beis Yaakovs will not accept a student if a family member wears pants or a married mother/sister does not cover her hair.
The reason they (the BY OTDs) frequently come back is not because they felt a connection to what their school offered them. It’s oftentimes because someone else from the community (in my case Modern Orthodox Rabbis and women) reached out or because they realized there are other drachot than that of Beis Yaakov. Or, most commonly once a person has settled down they will return back to what is most comfortable and familiar, and back to a similar lifestyle to which they were raised; it’s just easier to fit in that way.
Additionally, having gone through the Beis Yaakov system and having left the path to find my derech, many Beis Yaakov girls have seen me as a safe person to talk to. There are more girls not keeping Shabbat and kosher than you might like to think. And many of them look more observant than the ones who are keeping everything properly. There are also large numbers of Chareidim who are playing the part but internally feel no connection, much more so than by the Modern Orthodox.
This was not to generalize nor to suggest that there are huge numbers in any of the categories mentioned, however, it’s not ‘very very few’ from the Chareidi community, nor are they necessarily doing ‘more right’.