Here's why:

* Weak intermolecular forces: Nonmetals form weak intermolecular forces like van der Waals forces or hydrogen bonds. These forces are easily broken with relatively low amounts of energy, leading to low melting points.

* Covalent bonding: Nonmetals mostly form covalent bonds, where electrons are shared between atoms. These bonds are strong within a molecule but the forces between molecules are weak.

* Exceptions: There are exceptions like carbon (in the form of diamond) and boron which have very high melting points due to their unique structures and strong bonding.

In contrast, metals have high melting points because they have strong metallic bonds. These bonds involve a sea of delocalized electrons that hold the metal atoms together strongly.