Happy Xmas and New Year to you;
I'm not sure, but one of the reasons why those two US carriers are expanding in this market may be due to existing code shares between them and Latin American carriers. Because of these corporate relationships, certain countries may have allowed increased access to their airports.
Another reason may be that many Latin American governments may have previously limited access to foreign airlines so as to minimise their competition with local carriers, and give those local carriers a chance to fly those lucrative Latin America-US routes.
And the final hypothesis: is simply that the Latin American market has expanded rapidly in the past few years- more trade between an increasing no. of countries, and an increase in the no. of people (in Latin America) that can afford air travel. An example:
Three years ago BA flew only 3 times weekly from London to Buenos Aires-now it flies daily. Many other carriers are now flying to Latin America: JAL, Malaysia Airlines (EZE), South African etc...