As a former Jehovah’s Witness myself, and my wife a former Mormon I can clue you in on the possible reasons why somebody would want to join and maintain their membership in the respective religions.
1- When I was a JW [Jehovah’s Witness] it was our goal to go from door to door seeking out mainly biblically ignorant individuals. We would ask very simple questions provoked from the Watchtower magaine [ie… Is there a hell, is there such thing as God] how people responded to those questions gave us huge indications on where they were at emotionally, spiritually etc…
2- In my experience, people rarely Joined for theological reasons, it was mainly cultural. When they were invited to the Kingdom Hall [Church] they would usually be surrounded [love bombed] by others in the congregation. This would give a felling of acceptance and value. After which they would be invited to a bible study [which usually involved not reading the bible but reading from Watchtower literature
3- Once a person was baptized as a Jehovah’s Witness, the love bombing stopped. Now the person is expected to “tow party lines”
Once a person is fully immersed in the cultural and doctrinal influence of the Watchtower Organization, they are held captive and controlled by the Organization itself. The glue that holds the “Rank and file” members together is one of fear and conformaty to Organizational standards of ethics. If anybody disagrees or falls out of line, they will be quickly disfellowshipped and will be shunned by family and friends.
4- RECAP: Generally, people join the religion for social accpetance and to find value. Since the Watchtower seeks out biblically ignorant individuals [they will stop calling on your door if you challenge a jehovah’s witness at your door too much] rarely do people join for Theological reasons in the first place. Once they are in, they shortly discover that they are being told WHAT to think through fear and other manipulative control methods that are common for any high control cult group.
The people that escape this religion generally go two directions. All of them have a damaged sense of trust from being manipulated so I find many gonig the way of Atheism. They hate God and religion from what the Watchtower has done [taken away family and friends and equated God with the Organization itself] or they will repent and see God for who He really is and turn to Christ [as is the case with myself]
I do not see that much of a difference with Mormonism except LDS generally have more freedom with its members, they are not as extreme with shunning procedures [with the possible exception of some Utah Mormons] but still controlling nonetheless.