I completely understand your question....why do they go back. My thoughts on the matter are that, like you mentioned, they don't have a strong enough support system out of the organization. The question to ask is, What was their relationship with fellow witnesses when they were in the borg? Did they have some 'need' (usually emotional) that was met while a JW? If this is the case, as I believe a lot of dysfunctional people grab hold of the whole JW line of thinking and the 'family aspect' and 'sense of belonging' theory, this would be their reason for going back. They often have this 'need' to be met and because the organization was once 'comfort' to them, they give up hope outside the organization and 'fall' back into the JW line of thinking. When you think about it...it's a pretty good brainwashing job they do on people....it goes so far as to reel a lot of them back in if they don't 'cleanse' their minds while they're away. Another question is, How bad did they really want out when they left? If this was just an act of temporary rebellion or laziness as simply not wanting to attend, then this too will also pull them back in.
I keep myself in check periodically and ask myself these questions so I will know if I'm truly 'cleansing' my mind of the years of brainwashing so I don't get caught back up in it again.
anewlife....and so far....a better one!