Preservation of clay items
Its chemically stable oxide, thus non-combustible, does not rot, and fairly hard. 

Clay ceramics (just ceramic covers a huge swath of materials) are very non-reactive, and fairly hard, both are exactly what you want for preservation. The clay used in these ceramics is what is left when you chemically remove everything else from rock, it is usually the most stable components, mostly oxides. Earthenware clay in particular is often iron rich which makes it even stronger. once refired the clay transforms into a solid mass, essentially a man made rock.

When you look at what destroys artifacts most don't apply or apply the least to clay ceramics. 


Rot/decomposition: not an option for clays nothing organics to rot
Combustion. again not an option for clays, they are already oxides, under the right condition heat can actually make them tougher.
Erosion, this still destroys clay ceramics just like anything else but clay is fairly resistant. It is fairly hard and strong, some stones and metals are more resistant, and these are likewise more likely to be preserved. 
Corrosion, clay ceramics ar mostly chemically inert oxides, already in their lowest energy state, so they won't corrode or dissolve when exposed to water, so they survive better than most metallic metals, or ionic compounds. 
Theft vs rarity, ceramics generally were not valuable enough to be stolen unlike gold or gemstones or even plain stone, which would survive as well if not better. They were also common enough that that were basically all over a human habitation. If you had a human civilization you usually had hundreds of millions of ceramics. In the case of bricks the homes themselves might be made of it. So they were abundant enough that there was a good chance some would end up in the right conditions to be preserved. 


there is one other feature that applies to clay tablets. When a library full of books burns down the books are destroyed, when a library full of clay tablets burns down they clay actually gets tougher much of the time.