Short answer = personal integrity
He was very close to the leadership of this organization and personally saw the corruption which had been built up around itself.
I think its more important to take in why the leaders of this organization expelled him, rather than why did he leave himself.
Thats the game of power though, how its firstly created and further how its retained by those who have achieved it.
What mostly sparked his critical evaluation of the leadership was the obviously deceit of promoting 1975 by his
uncle F Franz, that was one false doctrine that resulted in many devout JWS to rethink what they were really
involved in, including myself.