In covalent compounds, bonds are strong between atoms in the molecule. But, bonds are generally weak between molecules, leading to a low melting point.
The explanation for this comes by considering the type of compounds that form covalent bonds.
Covalent bonds are generally formed between atoms that have a similar electronegativity.
As these atoms have a similar electronegativity, the resulting molecule has low polarity, and thus forms weak intermolecular bonds.
Weak intermolecular bonds = less energy required to break the bonds between molecules = lower melting point