Because dogs are a uniquely malleable species:
[B]ody size, hair length, fur type, nose shape, ear positioning, coat color, and the other traits that together define a breed's appearance are controlled by somewhere in the neighborhood of 50 genetic switches. The difference between floppy and erect ears is determined by a single gene region in canine chromosome 10, or CFA10. The wrinkled skin of a Chinese shar-pei traces to another region, called HAS2. The patch of ridged fur on Rhodesian ridgebacks? That's from a change in CFA18. Flip a few switches, and your dachshund becomes a Doberman, at least in appearance. Flip again, and your Doberman is a Dalmatian.
The reason humans are much more uniform:
In nature, a physical trait or disease state is usually the product of a complex interaction of many genes, each one making a fractional contribution. Height in humans, for instance, is determined by the interaction of some 200 gene regions.