Here's why:

* Electronegativity difference: Fluorine is much more electronegative than chlorine. This means the fluorine atom in HF pulls the shared electrons much more strongly towards itself, creating a larger partial negative charge on the fluorine and a larger partial positive charge on the hydrogen. This results in a stronger dipole moment in HF.

* Smaller size of fluorine: Fluorine is smaller than chlorine, leading to a more concentrated partial negative charge on the fluorine atom. This further enhances the dipole-dipole interactions in HF.

In summary: The greater electronegativity difference and smaller size of fluorine in HF contribute to stronger dipole-dipole interactions compared to HCl.