Answer:
Because of the breeds
Explanation:
Like in humans, the dogs belong to the same scientific group, but through the passing of time, there has been some changes in their DNA, slightly differences that make the size, the hair, and another traits to be different.
In humans all the "breeds" suffer the same changes, that is the reason that spite we are homo sapiens, there are different groups of humans as the oriental, Caucasian, etc.
