I honestly think it is because the JW's do have some fine doctrines that are more scripturally accurate than 99% of the rest of Christianity. The trinity, hell, immortal soul, etc is all bollox...false dogmas...and the JW's I still believe are right to reject these dogmas. So those that still 'buy into it' are sucked in by the fact that the Society does actually hold really trump cards doctrinally. Not everything they teach is rubbish...to the contrary, a lot of what they teach is very defendable scripturally compared to what the other churches are teaching.
It's just a pity there is so much doctrinal pooh mixed in with the good as well, such as all their ridiculous 1914 chronology and related prophetic interpretations, their idolization of the WBTS and distortion of the FDS concept, the authoritarianism, and most worringly their disgraceful policies relating to shunning, blood and child abuse. But if you took all that pooh away it would actually be a pretty cool religion I reckon.
Most who still buy into it or still believe it wholesale are usually just ignorant, wilfully or not, of the true facts about these pooh things.