I doubt that a person's actual native intelligence has anything to do with their remaining a follower of the Jehovah's Witnesses. While Jehovah's Witnesses appear dull, complacent, and incurious, they do not seem to be any more so than other Fundamentalist groups.
I suggest that people in industrialized nations remain Jehovah's Witnesses (and other weirdo Fundamentalist sects) because they are essentially incompetent. This incompetence may be caused by genuine mental slowness, ingrained conditioning, long-term mental illnesses such as depression, shorter-term mental tragedies such as the death of a loved one, troubled married or family life, loneliness, or an inferior social or economic position. When an individual-- of whatever age, or level of education-- finds themselves validated in their own minds as intellectually competent to address the key issues of their life and beliefs, no Watchtower indoctrination can hold them back. There is simply too much evidence, in any direction one cares to look, to a person with open and easy access to a newspaper, a library, a museum, or a public television station that directly counters a Fundamentalist, and particularly a Jehovah's Witness, outlook.
It is the near-criminal task of the WTBTS, then, in Westernized lands to seek out those who are already incompetent or nearly incompetent, and then maintain and even encourage the incompetence of their members, as well as foster and demand the incompetence of their children.
(The subject of Jehovah's Witnesses holding, or actively seeking, leadership rĂ´les is different than those who are followers, and requires somewhat more complex analysis, as does the conversion of those who live in non-Westernized countries.)