Why did NASA decide to return the Starliner without astronauts Wilmore and Williams aboard?

To allow engineers thorough review of system performance data following in-flight anomalies.

The decision to scrub the capsule for crew occupancy was fundamentally rooted in a safety-first approach following the observation of technical hurdles during the Starliner's time attached to the ISS. Specifically, engineers needed extensive time to diagnose and fully understand the performance characteristics of affected systems, particularly the Reaction Control System (RCS) thrusters, under real spaceflight conditions and accumulated operational stress. By returning the vehicle without the crew, NASA prioritized an exhaustive engineering review of the in-flight data over adhering strictly to the original timeline for crew departure from orbit, effectively decoupling the crew's return from the vehicle's return timeline for this specific test segment.