Why are there 8 bits in a byte? In some ways that is a little like asking why there are 12 inches in a foot. That's just the way it is. At some point in time, the folks building computers decided this and it continues today.

But there is logic to it. Remember that an 8-bit color depth has 2⁸ or 256 colors, and that each of the RGB primary colors ranges from 0 to 255.  With 8 bits in a byte, you can represent 256 values ranging from 0 to 255, as shown here: 

  0 = 00000000

  1 = 00000001

  2 = 00000010

   .....

254 = 11111110