The issue is how much JW time is spent preaching and to what effect in terms of new recruits.
The 2017 figure from JW statistics is 13,880 hours spent in the field for one new baptism. (Thank you Giordano) That includes time spent on Bible studies with JW sprogs born in captivity--who are now something like an 80% majority with 20% of baptisms being new converts. Even this might be over optimistic, it may be a 90%10% split.
The "born-ins" will receive around say seven years indoctrination at one hour per week. This amounts to an average of around a mere 350 hours of preaching as a lifetime total Bible study for JW offspring who get to be baptised.
Relatively speaking it doesn't then take that many hours for a "born in" Bible study to get to the baptism stage as already mentioned so if the average hours preaching before one new baptism is 13,880 that must mean that for the remaining 20% converts, those figures are immensely more than the already high average of 13,880. I'm guessing 25,000/30,000 hours-- just for one convert!
From a practical aspect it would be cheaper for JW HQ to pay people to become one of Jehovah's Witnesses!
From a field service point of view, it is no longer worth going from door to door, it's nigh on a complete a waste of time. Who on earth spends this amount of effort for such dismal returns?
Perhaps this is why eventually most cults don't proselytise but just plod on in cloaked obscurity, breeding new recruits within their families, fighting invisible foes but smug with the feeling of certainty in their divine assumptions?