Only after the number of printable characters had been greatly increased in 1967, making impossible the conversion to 6-bit character sets, and the new version was adopted not only in USA, but also internationally, by both ISO and CCITT, it became a necessity to have a byte size equal to or greater than 7 bits, in order to be able to store efficiently ASCII strings in computers.
From that moment on, the 8-bit byte size became a hard requirement for any new computer ISA, e.g. for DEC PDP-11, which was designed mostly during 1969 and it was launched in 1970.