@b0rk This is one of my favorite parts of this whole thing! I don't think there's a clear technical reason, given that we had 6-bit computers as well - even numbers are nice because you can split them into even nibbles, and powers of two feel familiar, but since bits aren't individually addressable, I don't think there are any material reasons to prefer powers of two.
I don't have any links on this, but one pressure keeping bytes from 10 bits is wasted space - how often are you going to use those top two bits? 256 is a pretty good balance, you have room for Latin text with some accents