[dmm]

I was going to make a snooty comment about 8 bits per byte naturally going with octal, but quickly realized that made no sense whatsoever. However, 8 bits goes well with hexadecimal. (Hex has digits 0 through 9 and then A through F, for a total of 16. That's probably as many digits as humans can comfortably handle.) So 8 bits per byte is 2 hex digits. A standard old-style keyboard had 26 lowercase, 26 uppercase, 10 numbers, 10 special characters, a space, a tab, and a return. That's 75 different characters, so ASCII code needed at least 2 hex digits. I'm guessing that was the driving force to have 8 bits per byte for PCs.

They could have used 3, 6, or 9 bits per byte, and then only used octal. But 3 is clearly too small to be useful, and 6 can't cover ASCII. (2^6 is only 64.) 9 bit bytes are overkill for ASCII.