Why does crystalline boron act almost like an insulator at low temperatures?

Crystalline boron acts almost like an insulator at low temperatures because it is a semiconductor.1

In semiconductor crystals, electrons are bound within their respective energy bands at low temperatures, meaning they are not available for electrical conduction. Specifically, electrons occupy the valence band, while the conduction band remains empty, separated by an energy gap. Because there is insufficient thermal energy at low temperatures to excite electrons across this gap into the conduction band, the material cannot effectively conduct electricity.21

As the temperature increases, thermal vibrations provide enough energy to break some covalent bonds, creating free electrons and holes that can participate in conduction, which is why boron's conductivity increases at higher temperatures.21