Because in JPEG, if you choose monochrome, the camera only records one channel of information, with a maximum of 256 shades of gray (from black to white).
So, if you want to shoot black-and-white, what are the things you should think about? First, you should leave yourself the option of going back to color. What’s that mean? If you shoot JPEG photos, DON’T switch your camera to monochrome. Instead, do a black-and-white conversion from the color image afterwards, in software. Why? Because in JPEG, if you choose monochrome, the camera only records one channel of information, with a maximum of 256 shades of gray (from black to white). However, if you shoot JPEG color, then you get three channels (RGB) of information, each one containing up to 256 shades of data. Which means WAY more data, which will  then give you more flexibility when converting to black-and-white later, on the computer.