Here's why:

* Lack of metallic properties: Carbon doesn't exhibit the typical properties of metals like good conductivity of heat and electricity, malleability (ability to be hammered into sheets), and ductility (ability to be drawn into wires).

* Formation of covalent bonds: Carbon readily forms covalent bonds with other atoms, sharing electrons to achieve stability. This is a characteristic of nonmetals.

* Brittle and non-lustrous: Carbon is brittle and lacks the characteristic luster of metals.

While carbon can form some compounds that exhibit metallic properties, like graphene, carbon itself is fundamentally considered a nonmetal.