intermolecular forces - Trend in the boiling point of the hydrogen halides

The boiling points of the hydrogen halides are as follows:

$$\begin{array}{cc} \hline \text{Species} & \text{Boiling point / }\mathrm{^\circ C} \\ \hline \ce{HCl} & -85.1 \\ \ce{HBr} & -67.1 \\ \ce{HI} & -35.1 \\ \hline \end{array}$$ ^{(source: Greenwood & Earnshaw, Chemistry of the Elements 2nd ed., p 813)}

My textbook writes that this is due to an increase in the number of electrons, which increases the van der Waals forces between molecules.

But the $\ce{HX}$ molecules are polar. Shouldn't the boiling point be determined instead by permanent dipole-dipole attractions instead? Since the dipole moment decreases going from $\ce{HCl}$ to $\ce{HI}$, the boiling points should actually decrease. Why isn't this the case?