The Yorkshire Terrier Club of America (YTCA) is the mother club of Yorkshire Terrier, and they set the standard for the yorkie. They encourage breeders to breed to this standard. The goal of any breeding should be to produce pups that meet standard, and standard is difficult to obtain, and produces a gorgeous dog that does look like a yorkie, but too many breeders think because there was a champion in the background it means that it ok to breed their dog. Nothing could be further from the truth, and most yorkies born, shouldn't be breed. Most breeders aren't trying to improve the breed, but are in the breeding "business." Whether a puppy mill or byb, these breeders don't care if the dog they breed is even close to standard. Some even take dogs that don't meet standard, and pass the fault as a positive, for example, in the case of Parti's and Chocolates. The YTCA believes that these dogs should not be bred, but placed in pet homes. Business breeders tend to market these dogs as "rare" and try to act like they are worth more than a yorkie who meets standard. Of course these breeders will deny they are in it for the money, and just think unusual colors are pretty, but then why charge more? There is a lot to learn about why breeding to standard is important to the breed, and why a breeder shouldn't breed for what is "popular" at the moment, but the primary reason breeding to standard is so important it that it's the only way to ensure a yorkie will still look like a yorkie 100 years from now. Here's a link to the YTCA and description of the standard.
Yorkshire Terrier Club of America
Another reason, Yorkies looks so different from each other is because Yorkies have the ability to grow long hair, there are many different hair styles they can wear, and changing the hair style can make a huge difference in appearence. Most dogs can't get that many different styles, because their hair is short.