What makes people choose a specific breed over others or the mixed breed? Visual impact is by far the first and most obvious factor. Some people who may know someone that has a breed which has impressed them temperamentally go that way. I have met many guardians who chose their dog after getting to know a friends dog. They became anamored with an individual or multiple dogs and decided that this was the breed for them.
The more popular visual decision is the worst way that someone can choose a dog. Dogs may be the complete opposite of what they look like. Take my breed; the standard poodle who look very much like an empty headed fashion model. Far from this visual appearance the poodle is highly intelligent, a superior athlete and all round amazing utility dog that will participate in any sport, performance or activity required. The poodle is so intelligent that they sometimes easily outsmart their guardians.
Some of the cute small breeds can be the most difficult to handle which contradicts their appearance. Take my little JRT; small, compact and adorable but this cute exterior easily camoflages an easily triggered, high drive and very dominant little breed. These dogs are often "dumped" once the unsuspecting guardians find out what they have. Sadly these mistakes could be easily be avoided by doing some research.
Even within a litter; a guardian may choose the worst puppy for their lifestyle. By choosing a dog by visual impact alone you miss the behavior signs. Even within a mixed breed litter there is a vast difference in puppies. Temperament testing alone does not create the final dog but environmental lifestyle impacts as well.
Research, research, research.