seems hard to pin down exactly, but in this Computerphile interview the gist seems to be "if you get to the point where you want a distinct code for upper- and lowercase characters, digits, and a few punctuation symbols, you land a little north of six bits, an odd number of bits would be annoying to implement in hardware, so let's go for eight."

https://www.youtube.com/watch?v=vuScajG_FuI&t=184s