I think wanting to believe is a major factor, and then fear of being disfellowshipped keeps them in the gullible stage afterward.
First they dangle the carrot(paradise,everlasting life, perfect health, and on and on0. Then get them to commit to a Watchtower/bible/indoctrination study, keep inviting them to meeting where peer pressure, and love bombing will drag them further into the WT mind control system and in order to keep them useful/busy give them lots of things to do and threaten subtly with DFing if they stop believing.