I completely agree with DaasYochid. It is useful to ask people who are off the derech what led them to go off the derech. It is also useful to ask people who observed them, their rebbeim, their families, etc. Every answer you get will be a useful data point but every answer will also be based in large part on the biases of the person giving the answer. Additionally, there are probably almost as many reason for going off the derech as there are people who went off the derech.
My highly biased answer, which applies to everyone who went off the derech: Because they were not getting enough out of frumkeit, at least in the short term, to keep them on the derech.