Here's why:

* Van der Waals forces are the weakest type of intermolecular force. They arise from temporary fluctuations in electron distribution around molecules, creating temporary dipoles.

* Dipole-dipole interactions are stronger than van der Waals forces because they involve permanent dipoles in polar molecules.

* Hydrogen bonds are the strongest type of intermolecular force, involving a special dipole-dipole interaction between a hydrogen atom and a highly electronegative atom like oxygen, nitrogen, or fluorine.

Boiling point is directly related to the strength of intermolecular forces. The weaker the forces holding molecules together, the less energy is required to break them apart and cause a substance to boil. Therefore, substances with only van der Waals forces have the lowest boiling points.