Basically because there's 2 major ways to do it: the Windows way and the Unix way (UTF-8). Unicode has the concept of encodings and it doesn't tell you which one to use.

The Unix way is winning on the web, and I think Microsoft has made some moves toward UTF-8, but I don't understand what they are exactly:

https://en.wikipedia.org/wiki/Unicode_in_Microsoft_Windows#W...

JavaScript and Java inherited the Windows way. Go and Rust use the Unix way (and apparently OCaml too). Python supports both which some say is a needless source of complexity, but it is flexible if you know how to use it.