The Imperium has strict laws controlling traffic in artifacts. Artifacts cannot be legally sold unless registered with the Imperium. The process of registration allows the Imperial authorities to analyze each item; rubble is registered and allowed, while more interesting items are confiscated and kept for deeper analysis. [36]
Side note: The canonical description of the extreme scarcity of working Ancient artifacts and the Imperial habit of confiscating any such seems to me to be contradicted by the existence of the artifact collector described on p. 10. Said collector has "an extensive collection of Ancient artifacts". These could, of course, all be of the rubble category, except that among these artifacts are FIVE operating shimmersuits "which he has accumulated over the past few years", and he is actually willing to loan them to the PCs (In exchange for a share of any finds they make, but still).
Hans