I'm not surprised that a large number of former Witnesses leave religion altogether. As others have mentioned, the Watchtower basically poisons the well. We are taught that other religions are full of money-grubbing pedophile clergy who teach lies (and to some extent, this is true!).
Many Witnesses that I grew up with are not very "religious." We always mocked "holy rollers" and "Jesus freaks" and got great pleasure out of proving their beloved doctrines (e.g., trinity, immortal soul) wrong. Many of us liked the JW religion more for what it was not rather than what it was.
So what happens if you take a member of a religion who is not really religious and believes that all other religions are bogus, and show him that his religion is wrong? Well, it seems reasonable that he would reject religion in general. Like someone once said: 'everyone's an atheist about most gods (Thor, Zeus, etc.)...I just take it one god further.' That's what a lot of Witnesses do vis-a-vis religion.
I've talked to Witnesses who, when confronted with holes in their theology, state: "well nobody else is right." My response: take away the "else" and I'll agree with you.
In my personal opinion, many religious people simply lack the humility to recognize that maybe we don't have all the answers. And maybe we're not as special as we'd like to think we are. That's a sobering thought and it takes some intellectual courage and humility to consider that possibility. But isn't it the more likely scenario?