Diamond is harder than graphite because of :

Diamond is harder than graphite because diamond has a more complex structure. Diamond’s structure is like many pentagons connected together, each pentagon sharing a side with another pentagon or each pentagon sharing a point with another pentagon. All the points are linked together in some way. Graphite’s structure is very loose, with its bonds forming layers. There will be one sheet of elements bonded together, but then another sheet of bonds of elements will lay on top of that, and there will be very weak bonds holding those sheets together.