Many who leave, don't get an adequate education on not only in WT history/policy but other topics (e.g. science, critical thinking).
While very much a topic in of itself, learning critical thinking is really about learning a process. Once mastered, all other topics/subjects become much easier to think clearly through. One can fill their head with all of the science they want, but if they did not learn to critically analyze the information first, what is learned can be easily forgotten, and/or replaced with something far less logical. Many well educated people, (at least in the past), still became converts because they never really learned critical thinking to begin with.
Thankfully, most universities today teach classes in logic and critical thinking, and many majors require it as core classwork.
Edit: Many anti-science groups, (not just the JWs), are aware of the above and pushing very hard to remove critical thinking from core curriculums at both the high school and university level. I wonder why???
d4g