It's because McCarthy, like most people of the time, was a barbarian by any modern standard, and at that time, the users of the language were almost without exception white, middle-class, non-"immigrant" (in the last 100 years or so) Americans. ASCII was good enough for them. Character sets weren't expanded to deal with the other 6 billion people on the planet until later.
There's no point in using a language that's going to exclude 95% of humanity.
