it looks like the 14th amedment was for formerly enslaved people in 1868. but Mexican citizens from territories annexed by the United States were given the option to retain their Mexican citizenship or become U.S. citizens. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, allowed them to choose their citizenship status within a year of the treaty's ratification. This decision was made possible after the Mexican-American War, which ended in 1848, and the subsequent annexation of Mexican territories by the U.S.. Wikipedia +2
Trump explains why he's ending birthright citizenship