Money even if they do get lots of people in africa and asia if they dont have money then they can not give much to the local kingdom hall or the www
Age when people get old they die and just to keep the numbers the same you need new people to come in or else the group gets smaller so rather than having 6 kngdom halls in a city you might get 5 and then 4 etc and of course when people die they cant give any more money (apart from wills)