Diamond is a crystalline form of carbon while graphite is an amorphous allotrope of carbon.
Diamond has a cage-like structure consisting of the regular tetrahedrons in which each carbon is bonded with four other carbon atoms by a strong covalent bond.
While graphite has a layered structure that consists of hexagonal rings. In graphite, these layers are attached to each other by week Vander Waal force of attraction.
Hence, due to the presence of weak Vander Waal forces in graphite, Graphite is softer than Diamond.