This likely ties back to Stephen Miller. This guy despite being descendant from immigrants with liberal family (who disowned him) is super into white nationalism and he gets off at the suffering of non-whites. He pioneered the kids in cages/family separation policy during Trump I and he directed ICE to be confrontational with immigrants and protestors during this administration.

The citizenship data is two-fold. It gives companies like Palantir more data to train on and also helps fascists like Miller determine who is a citizen, who isn't and lets them figure out how to exert maximal harm. Previously this meant neighborhood raids with tools like ELITE built on aggregate data. With the backlash of the raids, they will switch to quieter strategies. They may shut non-citizens out of banking, look for any organizations that help migrants out financially and shut them out. There have been murmurs of something similar planned using data acquired from hospitals.

Surveillance.

There's nothing more dangerous than someone with nothing to lose. Mass surveillance and authoritarianism is their hope to contain the population they're about to fuck over even more.