The keyboard and piano has always been the main instrument to explain the musical stave, it also provide the pitch for the 7 letters. Strings, brass and wind instruments can be used for pitch, but unable to demonstrate the 7 letters and the sharps and flats to be logically visible. The harp can provide both but is too expensive to be used as a teaching tool. The guitar too can do the same but is too confusing as it has 6 strings where each string is like another keyboard.... it is hard enough just to explain one, and also each string has a different starting pitch adding more to the complexity. So in the end it is back to the keyboard (which is one string of the guitar), and has the closest association for learning to read music tool. Even though the sharp and flat letters are in scientific terms to be of equal value with the white keys, they are shown as black keys and look squeezed amongst white letters. Adding to the visual complexities for child or adults to contend with when dealing with music theory beyond the key of C.