Here's why:

* Ionic Bonding: CaO has a crystal lattice structure held together by strong electrostatic forces between the positively charged calcium ions (Ca²⁺) and the negatively charged oxide ions (O²⁻). These ionic bonds are very strong.

* Strong Attractions: To melt or boil CaO, you need to overcome these strong ionic attractions. This requires a significant amount of energy, hence the high melting and boiling points.

Let me know if you'd like to know more about the lattice structure of CaO!