Yeast is a single-celled fungus that makes dough rise by feeding on sugar to produce carbon dioxide; sugar substitutes do not activate yeast. Yeast enables baking through the process of fermentation, of which carbon dioxide and alcohol are byproducts. Sugar substitutes lack the chemical properties and energy content necessary for yeast to feed upon. Thus, sugar substitutes do not rise, strengthen or imbue dough with taste.
Yeast is a unicellular fungus that exists throughout the environment. The mass-produced form lays dormant until mixed in water. When exposed to warm water and sugar (whether it is table sugar or the sugar in flour), yeast begins to feed. It then releases carbon dioxide and alcohol as byproducts.
The flour and water combine to form a substance known as gluten, which becomes stretchy as it is kneaded. The emitted carbon dioxide fills up and inflates air bubbles within the gluten. As a result, the dough rises. The yeast fermentation also strengthens the dough and creates flavor.
Sugar substitutes are completely different compounds from sugar. Often, they also contain less food energy than sugar. Yeast does not feed upon these substances. Consequently, the yeast does not ferment and is unable to perform the desired baking functions.