The reason this doesn't work is that previously the double-quote literal was a "string" type. The string type was, yes, just a sequence of bytes, but in an ascii-centric world that also mean text.

Python2 added unicode string literals that accepted unicode code points. Most APIs were happy to sloppily mix the two and generally work quite adequately.

Python3 then made the hard distinction between byte-string and unicode-string. Not an unreasonable position to take on the face of it. The issue is many python2 APIs were written from the perspective of "accepts string literal types", where that could be either bytestring or unicode string.

Now suppose you have a large codebase in python that spans the entire stack from database interaction, to webserver, to desktop application. All built on double quoted string literals. Accepting unicode strings in the places that needed that (user-facing places mainly, utf-8 bytestrings anywhere being stored on disk or sent over network)

Then you go to switch to python3, and suddenly all of your string literals are interpreted as unicode instead of bytestring / ascii sequences. So now you need to go through every place in your codebase that accepts strings as an argument and determine, "is this a user-facing string, or a utf-8 bytestring", because they used to be basically the same thing, and now they aren't.

It's not "difficult" really, it's just a pain in the neck.