Reliability is crucial to space applications. The software never crashes or misbehaves, despite the fact that both systems were designed to run nonstop for weeks or even months. Performance is the definitive factor in measuring real-time critical software. That is, they receive data from the spacecraft, display it and send it back to the satellite, all in real time. Everything runs within the specified communications rate between craft.
Software portability is of vital importance for upgrades and applications enhancement.
Portability among UNIX flavors can be done quickly, preserving expandability and keeping manpower costs down. This is not true for other non-UNIX operating systems. Nowadays, space applications often lack the funds needed to buy costly licenses. Linux is a zero-cost operating system, which provides true affordability. It can be copied as many times as desired, keeping license costs and royalties low.