It's simple, look at the thread title.
Because at the lower level, they (not just Mexicans) are entering our country illegally and taking our jobs. At the higher level manufacturing plants are moving south, taking our jobs with them.
There are a lot of people out or work right now that would love to take the place of that illegal working on the construction site. But that's not going to happen because the contractor does not pay taxes on the illegal... making him much cheaper.
There is a lot of hard feeling going around right now.