Diamond is covalent, yet it has high melting point. why?

It is because diamond crystallize on what we call atomic crystal structure, which means that when it is in a solid state, there are atoms held together by strong covalent bonds.
In diamond, every carbon atom connects with four more carbon atoms with single sigma bonds.
Atomic covalent bonds have extremely high energies, which means very high energies must be used to be able to break them apart (melt them).

Diamond has a 3D network involving strong C-C bond which are very difficult to break and,in turns has high melting point.