Python 2 had an inherent flaw with character encoding. While passing data around, strings and bytes were treated as the same thing, until they weren't the same thing, at which point your software broke in horrible and unexpected ways.

Python 3 fixed it, along with some other less drastic changes. Despite a gradual and ever ongoing migration to python 3 by every major library (http://py3readiness.org/ https://python3wos.appspot.com/), a lot of HN posters love to talk about how they'll never use python 3.

The secret is: no one cares. People still use java 1.4.2 and that hasn't impeded Java.

Python 3 was a large and backwards capability breaking change that required people to learn some minor new behaviors when writing code, but it has been and will continue to be the future.