It's been said that often people have previously encountered some tragedy in their life, or change or stress, that predisposes a person. A death of a love one. Moving to an area where they've few friends.
People don't plan on becoming JWs. It is a low and slow process. They are introduced to teachings that make some sort of sense, they decide to learn more without thinking they will commit to anything. They are not exposed to contrary information sources and are warned against it.
They might not believe everything, but they make a few changes to their behavior, and their beliefs soon align to their behavior. That starts a cycle of modifying their behavior to more align with their beliefs, which aligns their minds to the beliefs ever more, ect...
They like getting attention from JWs visiting them...that leads to meeting invites where they are lovebombed. They mostly only see the positive, public face. It's hard to reconcile anything negative (however true) with this lovebombing. Even if they don't agree with some things initially, all they see is nice, "loving" people and they want more of whatever it is they feel they have that makes them that way.