Here's why:

* Insulators have a large band gap between their valence and conduction bands, making it very difficult for electrons to move freely and conduct electricity.

* Conductors have a small band gap or overlapping bands, allowing electrons to move easily and conduct electricity.

* Semiconductors have a band gap that is intermediate between insulators and conductors. This means that they can conduct electricity under certain conditions, such as at higher temperatures or when impurities are added.

Boron has a relatively large band gap, but it's not as large as insulators like diamond. This, along with its unique crystal structure, makes it a semiconductor.

It's important to note that boron is a poor conductor at room temperature, but its conductivity increases significantly at higher temperatures.