– There are a few reasons. The biggest of these is the resulting file size compared to 16 bit, and what this would mean for how much music could be stored on a compact disc (if the compact disc format were 24-bit capable) or other types of media. Thus, 24-bit files require considerably more memory than do 16-bit files. Granted, many modern recordings are done using 24-bit resolution, but only 16 bits are supported in the compact disc format.
– Another compelling reason is that since 24-bit files are so large compared to 16-bit, when compressed their size would still be huge – this would be a great limitation for portable music players, and frankly, much of the benefits would be lost on inexpensive ear bud-style headphones.