why not say canine?

The same reason I said "DEA agent" and not "an adult of working age" - the specificity of the role matters. I used a metonym for the animal as the police departments that work with dogs are called "K9 units"

A "canine" is any kind of dog. A "K9" is a very specific type of dog used in US law enforcement, specially trained to act as if it smelled something when it sees a special signal from its handler.