Using UTF-8 by default is easier on new programmers
Python is one of the most popular first programming languages.
New programmers may not know about encoding. When they download text data written in UTF-8 from the Internet, they are forced to learn about encoding.
Popular text editors like VS Code or Atom use UTF-8 by default. Even Microsoft Notepad uses UTF-8 by default since the Windows 10 May 2019 Update. (Note that Python 3.9 will be released in 2021.)
Additionally, the default encoding of Python source files is UTF-8. We can assume new Python programmers who don't know about encoding use editors which use UTF-8 by default.
It would be nice if new programmers are not forced to learn about encoding until they need to handle text files encoded in encoding other than UTF-8.