Here's why:

* Silicon and oxygen are both nonmetals. Covalent bonding typically occurs between nonmetals.

* Sharing electrons. Silicon and oxygen share electrons to achieve a stable octet configuration in their outer shells. Silicon has 4 valence electrons, and oxygen has 6. Each oxygen atom shares two electrons with the silicon atom, and the silicon atom shares one electron with each oxygen atom.

* Strong bonds. Covalent bonds in SiO₂ are very strong, resulting in a high melting point and a hard, rigid structure.

Let me know if you'd like more details about covalent bonding!