The perception that the rulers of the country are a bunch of kleptocrats who are likely to be ousted by another set of kleptocrats in short order.

Generally however as long as the artifacts are in hands of a museum they're generally safe no matter how unstable the host government is. The only exception is when certain religious extremists gain power and go on an spree of cultural destruction like in Syria.

If there's one thing we British know about, it's valuable artworks disappearing from their homelands under deals of questionable legitimacy.