It's sad but very easy to understand:
When "the truth" is all you know and you "fall" into "sin", you kid yourself - and others - that, underneath it all, you are a far worthier individual in Jehovah's eyes by shunning those who have deliberately and willfully repudiated the truth. A brother or sister disfellowshipped for immorality can say in earnest, either to Jehovah or the elders, "See! I am not like an apostate!" And say it without trace of blushing.
And, let's face it, for untold numbers of people, the JW message has "saved" them from an even worse lifestyle. All their personal experiences indcates that, once they get their act back together, they will go back. It's called having your cake and eating it too. Some in fact do go back (the revolving door syndrome),, others keep kidding themselves they will go back whilst continuing to abhor deliberately criticizing Jehovah's organization. These individuals are kind of pathetic, never quite in nor out of the organization.