Here's why:

* Ionic Strength: Mg²⁺ has a higher charge density than Na⁺. This means that the attraction between Mg²⁺ ions and Cl⁻ ions is stronger than the attraction between Na⁺ ions and Cl⁻ ions.

* Lattice Energy: The stronger the ionic attraction, the higher the lattice energy. MgCl₂ has a higher lattice energy than NaCl.

* Boiling Point: A higher lattice energy means more energy is required to break the ionic bonds and transition the compound into a gaseous state. Therefore, MgCl₂ has a higher boiling point than NaCl.

In summary: The higher charge density and stronger ionic attraction in MgCl₂ result in a higher lattice energy and ultimately, a higher boiling point.