Here's why:

* Dipole-dipole interactions: While HI has a polar covalent bond (due to the electronegativity difference between hydrogen and iodine), the difference is relatively small. This results in a very small dipole moment, making dipole-dipole interactions negligible.

* Hydrogen bonding: Hydrogen bonding requires a hydrogen atom bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. Since iodine is not highly electronegative, hydrogen bonding is not present in HI.

* London dispersion forces: These forces arise from temporary fluctuations in electron distribution, creating temporary dipoles. All molecules exhibit London dispersion forces, and they are the dominant force in HI due to the large size of the iodine atom.

Therefore, the primary intermolecular force in HI is London dispersion forces.