Needy people are attracted to those who are confident, and JWs behave in a confident way. The WTS is very confident in their message (even though it's wrong!) and that gives a needy personality an anchor to cling to. Neediness is a form of social anxiety, and the (false) confidence of the WTS and JWs helps an anxious individual to have some manner of tending to their anxiety, helping them focus on something other than themselves and their fears. The WTS offers them hope that if they just do such-and-such, they'll learn to be like so-and-so, who is confident and able to get up on the Platform™ and speak to an audience, or go Door-to-Door™ and speak with authority to complete strangers.
They could learn the same skill set by going to Toastmasters, but Toastmasters doesn't recruit Door-to-Door™.