One reason is the complexity. In terms of notation, rhythm, harmony, structure and technique there's no other genre that is fit for comparison.
Classical and Jazz have also been around for an extremely long time (Lindy Effect) compared to most music today.
Works that are written for classical and jazz music have also been written not only by famous, talented composers but also composers who were masters and virtuosos of their instrument (piano).
Lastly, if you follow the classical periods (baroque, classical, romantic, etc.) all the way to jazz you'll be able to trace the evolution of piano playing and history.