Why are chocolate labs never an option for this breed group? I've watched Westminster for years (13 years), the same age as my chocolate lab, and have only seen yellow or black labs.

All the colors of labs are shown together in their breed. It's so competitive in the lab breed that a chocolate lab has just never won their breed competition at Westminster at least since you have been watching.

Labs all fall into the Sporting group. AKC does not differentiate on the color of a lab it is the same breed. It is up to the breed judge on whom he thinks should go BOB in each catagory.
A chocolate lab is still a lab and no different then a yellow or black one.