Here's why:

* Disaccharides are carbohydrates made up of two monosaccharide units joined together by a glycosidic bond.

* Monosaccharides are simple sugars containing carbon (C), hydrogen (H), and oxygen (O).

* Therefore, disaccharides also consist only of carbon (C), hydrogen (H), and oxygen (O).

Nitrogen is found in other biomolecules like proteins and nucleic acids, but not in disaccharides.