Here's why:

* Intermolecular forces: Boiling point is largely determined by the strength of intermolecular forces between molecules. The stronger the forces, the more energy is needed to overcome them and cause a change of state.

* London Dispersion Forces: AsH3 and PH3 are both nonpolar molecules, so their primary intermolecular forces are London Dispersion forces. These forces increase with the size and polarizability of the molecule.

* Size and Polarizability: Arsenic (As) is larger than phosphorus (P) and has more electrons. This means AsH3 is larger and more polarizable than PH3.

* Stronger Dispersion Forces in AsH3: The greater size and polarizability of AsH3 lead to stronger London Dispersion forces between AsH3 molecules compared to those between PH3 molecules.

Therefore, it takes more energy to overcome these stronger forces and boil AsH3, resulting in a higher boiling point.