Without a shadow of a doubt the number one reason has to be...
LIES - Coverups of failed prophecies, changed doctrines, sanitised history of organisation and bogus reasons behind changes in policy to name but a few
2. Conditional friendships - real friends stick by you regardless of changes in what you believe
3. Control - over thought, emotions, behaviour, dress, relationships, sexual life, information, career, education, health
4. Severe punishment for showing integrity and exposing lies - disfellowshipping/shunning leading to psychological damage and breakdown of familiies/friendships and in extreme cases suicide
5. Mistranslation of the Bible
6. Secret judicial committees that do not deliver justice and pry into issues that are none of the elders' business
7. Unqualified men (elders) giving advice that can cause irrevocable damage in the areas of marriage, mental health, relationships, careers, education, family, children
8. No real joy in worship
9. Individual talent suppressed leading to: lousy music at services, drab sermons, boring assemblies, pathetic attempt at charity, no youth program, archaic preaching methods
10. Relentless routine and high expectations that lead to mental and emotional breakdowns/damage
I could go on but you did say the TOP TEN.