Hi Cano,
The game pauses when you leave its window because that is the sensible thing for it to do.
The ships have no autopilots. If it does not have your full attention, it is unsafe for it to proceed.
A more practical reason is that the processor and ram requirements are very high in many of the missions. Attempting to run it in the background while other demanding programs are running would inevitably lead to crashes, cries for help from technical support, and demands that QuestViewer be scrapped.
The demo is not as demanding, so there is less chance of a crash.
Regards,
Marty