“Those countries should have incentive enough”
In terms of national interest, yes. Unfortunately, politicians (even more than most people) normally do what is in their interest, rather than than their national interest.
Those are the incentives that need to be managed to get the outcomes that you want. Trump seems to be very much aware of these pragmatic factors, and doesn’t move until he has the intel on the decision makers, and has lined up the leverage on them. Then he packages it to make it psychologically easy for them, with face saving pleasantries.
I believe that there will be some improvement in the immigration flow out of this effort, but a lot more will need to be done on individual incentives (like welfare, work, citizenship, and chances of deportation); as well as domestic law, policy and security - the Goodlatte bill, a wall and welfare crackdown.
I get the impression the Creole elites of Latin America are clearing their land of “surplus peasants” in the same manner England tried to clear Ireland for use as grazing land for livestock...