The Nationals can do it

Popular reason for stopping overseas work, this one.

And there is a lot of truth in it in many places, especially in the case of primary evangelism. Clearly the gospel will often gain a better hearing when it comes from someone from "down the road" rather than being seen a foreign import. 

But:

Even when there are many Christians in an area, and maybe a growing church, there are often areas where overseas help is appreciated: e.g. theological training, technical expertise.

And there are many countries where the national church is very tiny. (See Morocco, for example) In these places a well trained and willing-to-learn Christian from overseas can be of real benefit.