The modern piano has 88 keys because it is a compromise between the range of the human voice and the range of the harpsichord, the dominant keyboard instrument of the Baroque era.

The earliest keyboard instruments, such as the clavichord and harpsichord, had a much smaller range than the modern piano. The clavichord, which was developed in the 14th century, had a range of about four octaves, while the harpsichord, which was developed in the 16th century, had a range of about five octaves.

In the late 18th century, the piano was invented, and it quickly became popular due to its ability to produce a wider range of dynamics and expressiveness than the harpsichord. The earliest pianos had a range of about five octaves, but as the instrument evolved, the range gradually increased.

By the 19th century, the piano had a range of about six and a half octaves, which is close to the range of the modern piano. However, it was not until the 20th century that the modern piano with 88 keys became standard.

The 88-key piano allows for a wide range of musical expression and can accommodate the range of the human voice, which typically spans about three and a half octaves. It also allows for the performance of a wide variety of repertoire, including works from the Baroque, Classical, Romantic, and modern eras.

In conclusion, the modern piano has 88 keys because it is a compromise between the range of the human voice and the range of the harpsichord. The 88-key piano allows for a wide range of musical expression and can accommodate the range of the human voice, as well as a wide variety of repertoire.