Why are all monosaccharides reducing sugars but not all disaccharides?

An aldehyde or ketone functional group is required for a sugar to function as a reducing sugar; I will only discuss aldehydes, but the same applies to ketone sugars. Monomer sugars have an equilibrium between their aldehyde form and what is called their hemiacetal form (linear form and cyclic form). This means that the hemiacetal carbon can revert to an aldehyde, which permits the sugar to function as a reducing sugar. All monomer sugars have this equilibrium (Ketone to Hemiketal...if we are talking about ketone sugars).

A disaccharide typically forms when two glucose units, for example, are linked by a bond between the hemiacetal of the first glucose and the 4′hydroxy of the second glucose. The hemiacetal of the first sugar is converted to an acetal (no equilibrium with straight form, so it cannot reduce), leaving the hemiacetal of the second glucose intact, allowing this end of the disaccharide to still reduce.

A dissacharide in which both hemiacetals have been converted to acetals is formed when one hemiacetal end combines with another hemiacetal end. Since there is no longer an equilibirum with the straight (aldehyde) form, you have lost your ability to reduce.

Using sucrose as an example, you lose the equilibrium with the aldehyde/ketone and your ability to reduce sugar when glucose hemiacetal links with fructose hemiketal, creating this peculiar acetal/ketal hybrid. However, there is no longer any hemiacetal/hemiketal.

Monosaccharides are reducing sugars because they have a free anomeric carbon that can undergo oxidation, reducing other compounds in the process. Disaccharides, on the other hand, may not be reducing sugars if the anomeric carbon of one or both monosaccharide units is involved in a glycosidic bond, which removes its ability to act as a reducing agent.