The answer to this question can be found in the last pages of Chapter 11 of Harvest of Empire. I do not know the page numbers because I only have this in Kindle form, but the discussion of this topic is the last thing in Chapter 11. In these pages, Gonzalez lists the three major factors and discusses each at length.
The first factor that Gonzalez discusses is what he calls “the catastrophic economic crisis in Latin America.” Immigration typically happens in part because immigrants are “pushed” out of their home countries by some sort of negative factor. In Latin America, that negative factor is a weak economy. Gonzalez says that Latin American economies will remain very weak and that people will therefore be motivated to leave their countries and try to come to the US.
Gonzalez’s second factor has two parts. First, he notes that Latin America is much closer to the United States than Europe is. Because of the relative proximity, it will continue to be fairly easy for immigrants from Latin America to reach the US. Second, Gonzalez argues that Latino immigrants are more suited to come to America than the old European immigrants were. He says that Latino immigrants are more likely to have been city dwellers, are more highly educated, and have been exposed to much more of American culture before coming here. For these reasons, they are more likely to be able to cope with living in American cities and will not feel the need to return to their home countries as much as European immigrants did.
Finally, Gonzalez says that Latino immigrants will keep coming because the American economy needs them. (He actually lists four reasons, but the last two can be combined in this paragraph.) Gonzalez says that immigration is really driven by the needs of the US’s capitalist economy. As the white population ages, the US economy will need more workers to replace the retiring white workers. Because of this need, it will continue to attract or even recruit more workers from Latin America.
For these reasons, Gonzalez predicts that immigration will continue and the Latino population of the US will continue to rise.