Most jws are desperate, desperately broke and no hope except of a new system. No friends out side the Borg so really nowhere to go. They hang on out of this desperation and so people don't leave in huge numbers like we would hope. Its the trickle effect, new ones leave after seeing the love bomb where off. Kids leave that are driven, the non driven stay. People who are treated badly also leave.
I think times will get rougher for the Borg and theyll make adjustments to try to stop the loses but in the end it will make it worse. Their doing some of this now but reality of the world is waking up even the stupid ones. Evolution was proven a fact back in 2004 so people are leaving religion in mass.