Satellites have predictable orbits and are otherwise tracked, but a stealthy balloon might go unnoticed and manage to surveil sites that don’t mask their sensitive activities when they think the sky is currently clear. It’s possible that the ballon is performing signals intelligence gathering, or something else satellites at altitude can’t do.

Also, the perception (true or otherwise) that the US is not fully able to control its national airspace is something China can make propaganda out of.