Here's why:

* Ionic Bonding: MgS has an ionic bond, where electrons are transferred from magnesium to sulfur. This type of bonding leads to a rigid crystal structure with limited electron mobility.

* Limited Electron Mobility: The limited mobility of electrons is crucial for thermal conductivity. In ionic materials, electrons are tightly bound to the ions, making them less efficient in transferring heat energy.

* Low Thermal Conductivity: As a result, MgS has a relatively low thermal conductivity compared to materials with strong covalent bonding or metallic bonding, which allow for greater electron mobility.

In summary: Due to its ionic bonding and limited electron mobility, magnesium sulfide is not a good thermal conductor.