It occurs to me that for many, religious indoctrination of one form or another starts very early on. Even if one does switch religion at some point, the original assumptions and programming don't necessarily change much, if at all.
As OnTheWayOut mentioned earlier - the assumption is often that there is a 'True Religion' out there somewhere,'I just need to find it' or 'God' will show it to me. Unless one can step outside tihs framework and think critically it seems to me to be easy, even perhaps unavoidable, that a person would end up back at the beginning of the circular thinking. Unless the fundamental assumptions can be challenged progress will be difficult, if not impossible.
As OMG mentioned, WT does a good job of poisoning the well as far as other religions are concerned, so unless it is possible to break out of the circular reasoning I can see how difficult it can be for a person to shake off such a carefully crafted and medacious strategy as the JW teachings.
Once out, for whatever reason, one has an opportunity to rationally consider the full picture, and these days there is a great deal of information around to help that process. I suspect that many react fearfully and grab for the nearest religion that matches their programming to fill the void. It is clear though that at least some do not, and remain outide orgnised religion as either atheists, agnostics or 'spiritual' and that others go back into orgnaised eligion of one form or another but with perhaps a more balance view.