Why is the melting point of hydrogen iodide larger than that of hydrogen fluoride?
The increase in dispersive attraction between HI molecules is greater than the loss of dipole interactions. Only in $\ce{H2O}$ do dipole forces dominate (Intermolecular and Surface Forces, Jacob N. Israelachvili). Contribution of dispersion to intermolecular bonding for various molecules:


$\ce{CH4-CH4}$ - 100%
$\ce{HI-HI}$ - 99%
$\ce{HBr-HBr}$ - 96%
$\ce{HCl-HI}$ - 96%
$\ce{H2O-CH4}$ - 87%
$\ce{HCl-HCl}$ - 86%
$\ce{CH3Cl-CH3Cl}$ - 68%
$\ce{NH3-NH3}$ - 57%
$\ce{H2O-H2O}$ - 24%


What makes the contribution of dispersion so great is that it's non-directional and always attractive. Think of atoms as sticky spheres.