I don't know if this is the reason or if the causality goes the other way, but: it's worth noting that we didn't always have 8 general purpose bits. 7 bits + 1 parity bit or flag bit or something else was really common (enough so that e-mail to this day still uses quoted-printable [1] to encode octets with 7-bit bytes). A communication channel being able to transmit all 8 bits in a byte unchanged is called being 8-bit clean [2], and wasn't always a given.

In a way, UTF-8 is just one of many good uses for that spare 8th bit in an ASCII byte...

[1] https://en.wikipedia.org/wiki/Quoted-printable

[2] https://en.wikipedia.org/wiki/8-bit_clean