The ruling is because the texas law ran counter to federal law and treaty obligations - I cannot find the specific text, but likely this is beacuse the texas law denied the suspects some facet of due process, like an immigration trial.

Edit: Ah, yes, that would appear to be the case.

The measure would allow state law enforcement officers to arrest people suspected of entering the country illegally. Once in custody, they could agree to a Texas judge’s order to leave the country or face a misdemeanor charge for entering the U.S. illegally. Migrants who don’t leave after being ordered to do so could be arrested again and charged with a more serious felony.