Here's why:

* Organic Chemistry Definition: Organic chemistry is the study of compounds containing carbon, with the exception of a few simple carbon-containing compounds like carbon dioxide (CO2) and carbonates.

* Carbon's Bonding Ability: Carbon has the unique ability to form four covalent bonds with other atoms, including other carbon atoms. This allows for the creation of long chains, branched structures, and rings, leading to the immense diversity of organic molecules.

While other elements like hydrogen (H), oxygen (O), and nitrogen (N) are also common in organic molecules, carbon is the defining element.