Partly that, and partly dependencies. For example in one project we use VTK for visualization. The Python VTK wrapper until recently only supported Python 2.

Partly that, and partly dependencies. For example in one project we use VTK for visualization. The Python VTK wrapper until recently only supported Python 2. So the rest of the project is Python2/3 compatible, but to run the visualization we have to run the Python 2 version. VTK itself recently does support Python 3, but it's not in Debian/Ubuntu yet. And we'd have to upgrade our code to VTK 7. (Now 8, I believe, seems we've skipped a version.) So... lots of inertia all around, basically. The result is that in the meantime (meantime being like, 2 years), we compile Python 2 and Python 3 versions of everything. Not to mention that it's hard to justify breaking user's code that we aren't in control of just to tell them to fix their print statements -- though obviously we'll get there eventually.