Quality, hardness, and price are the main reasons. You can use a top lacquer for doing middle work but it is a waste of money since top lacquers are more expensive than middle, sometimes significantly more. Middle and top lacquers can differ in many ways but one of the main differences is in how hard they cure and therefore their durability. As lacquer is refined from a basic lacquer on up to a refined top lacquer the main process is the driving off of water and the subsequent concentration of the urushiol. Urushiol is what makes the lacquer form natural polymer bonds through curing. It is too simple to say you are paying more for just the increased urushiol in the lacquer since top quality lacquers also have oils and other qualities in their finish not related to just urushiol but all top lacquers contain a very high percentage of urushiol.