In almost every Python-related project I work on, we're developing in an exciting new language called "Python 2-compatible Python 3 subset". Twice as must testing, twice as much fun!
All the projects I've worked on moved to Python 3 and never looked back.
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.
When all dependencies were there, we made the switch.
I'm not blaming the person who builds the machine in the end, but the person that sets up the specification – they are doing a bad job.