Five reasons:
#1 They consider it a lack of faith to be having children deep in this time of the end. Very few couples are having kids. In fact, many people in their 30s and 40s are refraining from marriage to "seek first the kingdom".
I have seen a couple hide there pregnancy until the sixth month because they were embarrassed to be having children.
#2 Tons of pioneers/publishers are jumping ship moving to where the need is greater in Nepal, Taiwan, China, Mongolia etc. None of the young ones want to stay here because the atmosphere in the congs are restrictive, conservative and repressive.
#3 New growth is almost non-existent. The blood policy is a very sore topic here. Witnesses are well known here for their stance on blood and outsiders find it to be sick and cultish.
#4 Many of those who came in the 60's and 70's are dying off. There is an overall decline in population and an increasingly burgeoning elderly population.
#5 Japanese witnesses are crazy. Yes crazy. So many of them have mental problems and are battling severe depression. They combat this by giving 150% every day until one day they just... disappear and you never see them again. There really is no middle ground here. They are either in Samurai Christian mode or they just completely burn out and fall off the radar quickly.