Graphite is a good conductor of electricity because three of its four electrons are involved in bonding and 1 electron is free for conduction. But in case of diamond all four electrons are involved in bond formation. So no electron is available for conduction hence diamond is bad conductor of electricity. Graphite has layered structure where layers are held by weak van der wall force, that’s why it has soft in nature. In case of diamond, it has hard & compact 3D structure with strong forces acting gives it hard and brittle structure.

So, Assertion and reason both are correct statements but reason is not correct explanation for assertion.