Because if they were exposed for what the were, the public might see Jehovah (GB really) in a bad light. Obviously, it is better to let them hide!
They haven't been hiding them too well, though.
This guy was exposed. http://www.rickross.com/reference/jw/jw100.html
And this guy ... http://www.timesonline.co.uk/tol/news/uk/crime/article2316396.ece
And, yet again, another guy ... http://www.religionnewsblog.com/12106/no-jail-for-sex-abuser-in-case-that-cast-spotlight-on-jehovahs-witnesses
How the WTS can possibly think this issue is a big secret is beyond me!