Comment by KPGv2

Historical luck. Though "luck" is probably pushing it in the way one might say certain math proofs are historically "lucky" based on previous work. It's more an almost natural consequence.

Before ASCII there was BCDIC, which was six bits and non-standardized (there were variants, just like technically there are a number of ASCII variants, with the common just referred to as ASCII these days).

BCDIC was the capital English letters plus common punctuation plus numbers. 2^6 is 64, and for capital letters + numbers, you have 36, plus a few common punctuation marks puts you around 50. IIRC the original by IBM was around 45 or something. Slash, period, comma, tc.

So when there was a decision to support lowercase, they added a bit because that's all that was necessary, and I think the printers around at the time couldn't print anything but something less than 128 characters anyway. There wasn't any ó or ö or anything printable, so why support it?

But eventually that yielded to 8-bit encodings (various ASCIIs like latin-1 extended, etc. that had ñ etc.).

Crucially, UTF-8 is only compatible with the 7-bit ASCII. All those 8-bit ASCIIs are incompatible with UTF-8 because they use the eighth bit.