There are a couple of scenarios: - You want high performance and hence you want to leverage pypy, which has far better support for python 2 than for python 3.

There are a couple of scenarios: - You want high performance and hence you want to leverage pypy, which has far better support for python 2 than for python 3. - The libraries that you're using don't have good python 3 support. Flask used to have some problems with 3, but no longer does. Many companies have a lot of code written in Python 2 (dropbox), and hence using python 3 would not be a good idea.