Trying to boil down the reasons someone left to one single magic reason, is ridiculous. It's like asking someone why they joined the army, and thinking that everyone you ask will give the same answer.
I agree that there is not one single reason but the various reasons can be categorized. Using your joining the army example there could be numerous answers but in summary they might be categorized into just a few boxes.
Category 1 could be - Those who want to serve their country to be patriotic.
Category II could be - Those who want the training which the army can offer them and the monetary assistance they can receive to attend college when they get out.
Category III could be - Those whose family members made their careers in the army and so the person wants to continue the family tradition.
Categary IV might be - The person doesn't have any specific plans and nothing better to so he/she enlists in the army.
In this way, the infinity of reasons for joining the army can be condensed into just a few categories.
So it is for leaving staff in Scientology. The particulars of each person's experience are all quite different but a common thread runs through them all. That is why the "My Story in Scn" section on a board such as ESMB is so interesting and enlightening to read. Many of us have shared similar terrible experiences on staff in Scn but each experience is unique and has its own quirks and twists.
Hubbard told everyone that the only reason a person leaves is because he has committed overts against Scientology.
Also, he revealed to Bill Franks that the above was untrue and a person leaves only because he has an ARC break with Scientology.
Smila then suggested that the main reason for leaving was being over inflated, having too much pressure put upon you.
I offer two more categories which seem to run through many of the exit stories: The first category concerns "Promises Made but not Kept" and the second has to do with "Out Exchange" where a person is busting his balls giving everything he's got to support Scientology and in return he gets almost nothing in exchange or even negative exchange by being yelled at, humiliated and degraded.
I am suggesting that Smila's category and my two categories will cause ARC breaks so they fall broadly under the definition which Hubbard gave to Bill Franks. However, they are very large categories which run through many of the exit stories and seem large enough and common enough to justify separate categories. It's all subjective and just my opinion but it is usually helpful in attempting to understand some large phenomena to categorize it into generalized boxes.
Lakey