[SimplyUnknown]

The first edition of the C programming language was released in 1978, the second version in 1988. The first time something on Unicode was mentioned was also 1988, and the consortium was founded in 1991. UTF-8 was proposed in 1992.

Simply put, the book doesn't deal with alternatives to ASCII because there hardly were alternative text encodings at the time of writing in the western world, which is clearly the focus of an English book written by a Canadian and American.

Moreover, the point of the book was to propose the C programming language and showcase how the language works. It's not a book on best practices or how to use C in the real world today. There are other resources for that.