The problem with byte strings is that for any given byte string declared somewhere in the application, nobody knows the encoding of it to convert it into usable characters. So you end up making some class which ties the encoding together with the byte array. Or, standardising on some encoding. But if your application is going to be used by more than a single country, then what choice of encodings do you have if you want to support them all?

Even wchar_t has an encoding of course, it's just that it's always UTF-16. And UTF-16 still requires two units to encode a single character in some situations; the way it does it happens to be very similar to UTF-8.

I'm yet to see a system which plans ahead and uses UTF-32 for strings. It would take more memory, sure... but memory isn't that expensive.