For one Mexico has it's own part of the Americas ---also when the previous immigrants came, there was lots of empty land to homestead. Now there isn't and we already have cities that are plenty big enough. Where do you intend to put all the Mexicans you want to bring here? Should they be forced to build new towns --(except there is no vacant land left) or make our already big cities much bigger? Mexico City is huge ---if we bring everyone from there here ---that's a city far bigger than NY ---so where do they all go?