It is not that they will not think, or cannot think. It is fear, plain and simple. Fear of loosing their social circle, (big), fear of loosing their identity, (bigger), etc. Fear is a very low level motivator, that works at the lowest levels of Maslow's hierarchy of human needs very effectively. Since the lower needs generally need to be cared for first, (the second Safety level is where most of this fear happens), higher levels, (especially self-actualization where "facts" matter), are of no concern to the believing JW. This is science, and mostly psychology 101.
I know many very intelligent JWs who are not mentally lazy, and perfrom intellectually well in every other aspect of their lives. When it comes to questioning the organization however, the fear kicks in, and all rational thought is suspended.
Most of us did not "think" it through properly either when we were in. It usually takes an event that helps us in overcoming the fear to finally begin to question things properly. Even the "event" itself is more of a process, than a single event.
d4g