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.

Because they are legal under several international laws and our federal laws. And the vast majority of illegal immigrants be a travel, student, or business visa and just over stay. This law would just be a green light to harass brown people near the boarder. Which would be against the constitution!

I don’t know the specifics of the case but international law which the US is signatory to allows asylum seekers to enter countries via whatever mechanism (ie “illegal” is irrelevant). Not sure if that’s the issue here. But the mention of international obligations in the article makes me imagine it’s something similar

Murder being exclusive to the Fed isn’t in the Constitution but Immigration being the sole responsibility of the Federal Gov. is.