Why does diamond react less readily than graphite with the same reagent?

The difference in chemical reactivity between diamond and graphite is primarily a consequence of their distinct crystal structures.1

In diamond, every carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral array. This structure is exceptionally stable, and destroying it requires the rupture of many strong bonds. Consequently, the diamond lattice is highly compact and resistant to chemical attack.1

In contrast, graphite consists of parallel layers of carbon atoms arranged in fused hexagonal rings. While the bonds within these layers are strong, the interlayer distance is large, and the layers are held together only by weak van der Waals interactions. This open, layered structure allows for the formation of many compounds, known as lamellar or intercalation compounds, by the penetration of molecules or ions between the layers. Because graphite’s structure is more accessible and less rigidly locked than the dense, tetrahedral network of diamond, it reacts more readily with reagents.1