One reason, according to the Federal Government's indictment (PDF), why they consider the alleged corruption of the Armando Villalobos, DA of Cameron County, TX, to be a federal case (and why they consider his office to be a "racketeering enterprise" under RICO). Brownsville and Mexico are so intertwined that the Brownsville DA affects foreign affairs!
8. At all times relevant to this Indictment, the Office of the District and County Attorney 04 Cameron County. Texas. constituted an enterprise as that term is defined by Title 18. United States Code. Section 1981(4). which was engaged in and the activities of which affected interstate and foreign commerce.
9. The Office of the District and County Attorney of Cameron County, Texas, was physically located within the Cameron County Courthouse in BrownsviIIe, Texas. H was created by statute to execute and administer the laws of the state of Texas in a fair and unbiased manner As a District and County Attorney’s office on the international border, it was involved in various aspects of Interstate and foreign commerce including, but not Limited to, prosecution of individuals from Mexico and other countries and cases involving goods and services traveling in Interstate and foreign commerce. as well as matters involving a party from Mexico or another foreign nation, or a party with interested children and other family matters or heirs from Mexico or other foreign nations or other U.S. states, whose rights and interests were involved in the litigation.
The indictment (PDF) is in image format, the text above may have scanning glitches: