Most people didn't learn to work around unicode issues during the 2.7 time frame.

Many third party modules tended to explode spectacularly with encode/decode errors, when they were fed non-ascii strings, when used by people who write using all these funny characters.

Just the correct handling of strings in Python 3 alone is a hell of a reason to switch to it.

Let's not forget the joy of third party libraries that wouldn't fail, but rather silently convert, your data because they had hidden ascii asumptions...