I believe that 10- and 12-bit bytes were also attested in the early days. As for "why": the tradeoffs are different when you're at the scale that any computer was at in the 70s (and 60s), and while I can't speak to the specific reasons for such a choice, I do know that nobody was worrying about scaling up to billions of memory locations, and also using particular bit combinations to signal "special" values was a lot more common in older systems, so I imagine both were at play.