When you are an inmigrant and you go to an economical supperior country it means that:
1) you went there to make money, not to work half time to do the minimum money and pionering
2) you are not where they need you most: the preaching work is doing nothing in the developed countries, they grow in the poor countries. So they need people to stay in the poor countries.
3) when you are in a developed countries, most of the problems that you saw and suffer everyday are gone, so the hope for paradise are less strong (and less needed)
4) when you move to another country, you lose your ties to the congregation (your friends) and they have less control over you (remember, they use your friends as blackmail)
5) you will find out in developed countries that you need education. And problably you will have the opportunity (and that can affect your cult training)
6) in developed countries are far more ways to spend the weekend and have fun!
7) in developed countries the people have better education, so they can make yourself dumb in the doors (and, after a while, you awake to the fact that you are not the bible expert that you think you are)
8) if you go to another country and sucess, it seems that you have any talent that they need to use (for free) where is needed (in the poor territories).
So, yes... It's easy to see why the want that nobody moves to improve their situation. It's an increased risk to lose a member, maybe a productive one.