2 vs 3 is not a small change. and tools for helping you do things (like a 3+2 compatibility lib) get installed with pip... which is horrifyingly fragile. `pip install your-lib` -> you have no idea if you have the dependencies you require, because pip doesn't behave rationally. it often Just Works™, but when it doesn't there's no help and a massive minefield of potential problems.

I currently find some users have python2, some have python3, some both. It's fine in a controlled environment, or if your users are Devs, but a pain with normal users who I just want to send a script to.