Simple answer: because everything in computers starts at 0. Address 0 is the first byte of memory, etc. You can now ask, "why does everything start with 0"? I

Simple answer: because everything in computers starts at 0. Address 0 is the first byte of memory, etc. You can now ask, "why does everything start with 0"? I guess because it gives you nice round (base-2) numbers. With 4 bits you can have 16 different "numbers", either 0 to 15 or 1 to 16. However, representing the number "16" in binary requires 5 bits, so you need an additional bit for the same number of elements. So representing 4 bits as 0 to 15 makes more sense