1. No one has done a systematic study, and there are a lot of definitions to be played with. If some is raised Hasidische in Boro Park, and ends up being modern in New Jersey, are they off the derech, and in the reverse, are they BTs (i.e. the Young Israel kid who grows up to be a Boro Park adult)? Lines get to be very fuzzy, especially if you want a clear border between frum and not-frum, which in essence is what is needed for a neat BT/OTD discussion.
2. If you look at published Shailohs and advice columns, you’ll note that until the 1960s you were more likely to read about frum parents asking how to act when they visited their treff kids, and since then more likely to be about the BT kid asking how to deal with their non-frum parents.
3. When secular Jews study the matter, they focus on shul membership, which is problematic since many members of modern orthodox shuls are not frum by any definition, and many orthodox Jews don’t join a shul (especially in big cities, and especially among hasidim – traditionally being a member meant accepting financial responsibility for the shul as opposed to being someone who came regularly to daven there).