Main reason IMHO is too many other things in their life and not focused on their new job training, it is not just a full time job while in training, its a full time life, and if you cant do that you most likely wont make it.

However in my class I have no idea about the details of the individuals who did not make it.

But you did not answer how much turbo prop time you got, since you were never on the ATR?