Different breeds are intrinsically suited to the type of work that they do. They are not limited by their will, or their desire, but rather by their physiological make-up, which is breed (i.e. genetically) determined to a large extent. Quarterhorses make good sprinters because they typically possess many power-lifter type muscle fibers, and thoroughbreds make excellent cross-country horses because they possess 'marathoner' muscle fibers.
For more information, please read the story What Makes a Quarterhorse Different from a Thoroughbred