ThomasMore: 2. Competive hierarchy that keeps the rank & file engaged
I think this is a big part of it, and I think they are also reaching a breaking point. The requirement that everyone go door-to-door and the encouragement for some to do X hours each month must be dragging the growth to enough of a degree that they keep finding ways to make the preaching work less onerous. But it's one of the ways to keep people busy and building up enough of a time investment so that they see it as a sunk cost.
So, if you weaken the requirements too much, the rank and file might lose enough focus to start wondering why they are doing it at all. But if you keep the reins tight, some will wear down or resist. It's a tricky balance, and I'm not sure there is a level where it works without some fallout. Or, they will keep pushing things to one end or the other, thus losing people faster each time. One can only hope.