Here's why:

* Substrate: This term specifically refers to the molecule or molecules that bind to an enzyme (the catalyst) and undergo a chemical transformation. It emphasizes the role of the reactant as the "material" being acted upon by the catalyst.

* Reactants: This is a broader term that refers to any substance that participates in a chemical reaction. It's not specific to catalyzed reactions.

Example: In the enzyme-catalyzed breakdown of glucose, glucose is the substrate, and the enzyme is the catalyst.