It's relatively easy to go from one totalist system to another. If JWs become disillusioned, they're ripe to become "born agains." But the reason that more evangelical ("born again") Christians don't become JWs is due to the evangelical establishment's labeling certain groups as "cults." For them, a cult is not necessarily a high-control group, but simply one that claims to be Christian and yet disagrees with certain basic beliefs (such as the Trinity, etc.). So not only JWs, but others such as Christian Scientists, Mormons, Unitarians, etc. have been labeled cults in this way. Once a "born again" comes in contact with these groups or their members, the bells go off. The evangelicals claim to be evangelizing the "cults," but in reality they are insulating their own people.
The original appeal of C.T. Russell was to "consecrated Christians" of his day - people who today would be considered "born again." But when Rutherford decided to have an earthly class that would be less spiritual, this appeal stopped. Then the evangelicals began fighting back with their cult ministries. While the WTS has also tried to insulate its people by calling everyone else Babylon the Great, once people see that the organization is also Babylonish they become ripe for the plucking. So there is a reason why people seem to be going in one direction and not in the other.