Ionic compounds are formed through the transfer of electrons from one atom to another, which creates charged ions. Nonmetals, on the other hand, typically have high electronegativities, meaning they are more inclined to gain electrons rather than lose them.
When two nonmetal atoms come together, they both want to gain electrons to achieve a full outer shell. Instead of transferring electrons, they tend to share them, resulting in covalent bonds. This sharing of electrons occurs because neither nonmetal has a tendency to donate electrons, as would be required to form an ionic bond.
In summary, two nonmetal atoms cannot form an ionic compound because they both prefer to gain electrons rather than lose them, which is a requirement for ionic bonding. Instead, they typically form covalent compounds where they share electrons.