I agree w/ Blondie. People who are sad and depressed are looking for something, so they are ripe for the JW picking.
Most cults don't want to attract the depressed unless they are able to undepress them with their message, but JW's are a victim of their own recruiting methods. Depressed ones are looking for attention, JW's are looking for Bible WT Literature Students. A JW pioneer literature distributor can rack up hours on one depressed person.
Also, many cannot live up to the pressure of regular JW activity. The shepherding calls to tell them "Go to the 5 meetings, get out every weekend in the distributing work." - that makes more become depressed. Telling a momma not to talk to her child and grandchild, again that's depressing. So much about JW's works against them. WHY DO THEY SUCCEED?