There are more people wanting dogs than the shelters can supply. We have done such a good job of shutting down puppy mills, and reducing the number of pet stores that sell dogs and cats, that we are now importing dogs into the US to fill adoption demand.
Many people want a young puppy, not an adult dog that may have been a problem for someone else. Go to your local shelters and look for cute puppies… there are none. The shelters often are full of adult chihuahuas and pit bulls, and various older dogs. These can be terrific pets, but not appropriate for everyone.
Often a person wants a specific breed. This may be due to previous experience with the breed, or certain desired traits. They may also want a dog for a specific purpose, such as hunting, or for a home with rowdy young children. See #2, above.