Here's why:

* Bonding: Diamond has a very strong covalent bonding structure. Each carbon atom is tightly bound to four other carbon atoms, forming a rigid lattice.

* Electron Mobility: The electrons in the covalent bonds are tightly held and cannot move freely. This means they cannot easily carry an electrical current.

* Band Gap: Diamond has a very large band gap, which means a lot of energy is required to excite an electron to a conduction band and allow it to carry a current.

However, there are some exceptions:

* High-Pressure Diamond: Under extremely high pressure, diamond can become a conductor. This is because the pressure can alter the electronic structure of the material.

* Doped Diamond: Impurities (like boron or phosphorus) can be added to diamond to make it semi-conducting. This process is called doping.

Overall, though, diamond is considered a very good insulator in its natural form.