Imagine you need to go through a library every time your string includes or might include the letter "s" or "v". Basically you'd need to use this library for all your strings. But then you lose compatibility with 'normal' string type and need to be constantly aware of the difference. You might want to use some other library that doesn't support this Unicode library at all, etc. It quickly becomes a very painful world to live in.
As you might imagine, just having support for Unicode baked in the language is very nice. Defaulting to Unicode for all text is even better.
I can understand why Lua in particular doesn't come with Unicode support out of the box, being so small. My comment was written in response to the more general claim that Unicode support is a strange thing to consider essential in a programming language.
(According to http://www.bckelk.ukfsn.org/words/etaoin.html, s/ä and v/ö are comparable in frequency.)