Because of the lower wage demands of the Mexicans and ''taking jobs'' from the natives.
Explanation:
In the early twentieth century the United States were thriving. The industry was constantly growing and the country was becoming more and more powerful. Mexico, on the other hand, was in an opposite situation, with the country being in big problems with internal unrest, corruption, low development. This resulted in mass migration of Mexican into the United States. This was not seen very fondly by the majority of the Americans, with the main reason being the competition for the jobs.
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