No, I think what people are getting at is... why does program source have to be represented by only ASCII text? After all, if code is data, an IDE is just a data editor, where that data happens to be code.
I think it has been shown many times throughout history that notation matters. In fact, the entire concept of Arc is based on this principle. Otherwise, Common Lisp would be suitable.
Why does it have to be ASCII? No reason, The simplest answer may just be that the language writer feels that adding UTF-8, or UTF-16, support would be a waste of their time, or is below them. As for programming IN the language, it becomes a matter of what is easy for the programmer to type. In a way, this whole thing is a matter of deciding what is the lowest common denominator that one wants to support.