Their minds are closed because they are blinded by Satan.
You may have a point, but I think there's a more empirical explanation...
I recently talked to a higly intelligent, knowledgeable, and extremely well educated JW who claimed he critically looked at the JWs before joining.
We had a brief conversation about why he believed in the JWs as the Truth. Throughout his conversation I picked up on 5 key rationalizations that kept the dream alive in his mind, despite the irreconcileable facts that would suggest otherwise.
1. God must exist because of the unexplainable wonder of the universe and our surroundings.
2. The Bible must be the Word of God.
3. The Biblical understanding of the JWs is not and has not been perfect, but the "light keeps getting brighter".
4. Nothing but the JWs "even comes close" to properly representing the Bible.
5. The JWs teachings as a whole must be of God because it "all fits" when you tie them all together, despite perhaps not being able to fully reconcile individual topics.
As I aluded to in another post, it's more about the sly seduction of the "package deal" the JWs offer than a completely rational decision.