[shiroiushi]

The artifacts weren't in a stasis field when they were buried in the ground: they were decaying there too. Whether that was faster or slower than after being excavated and stored in a museum depends on the artifact's composition and the properties of the place it was stored. I guess in a museum, however, they're somewhat more susceptible to damage or theft since now people know about them, or they could be damaged or destroyed in a war.