For me, it's not enough resources (time) to test whether existing infrastructure would work with python 3 or it wouldn't (and something had to be fixed). I can't test it in production and it's loads of (sometimes hidden) code, most of which I didn't write.
I don't think we'll ever move... well not in a next year or two.
If I had to guess, the same thing that has some places still stuck on COBOL.
It's amazing how a lot of big financial institutions are still running on COBOL.
Not just financial institutions - I've seen it when consulting at older medium-to-large companies. They've got new mainframes and code they've been running with minor patches for 30 some years.
I find the mainframe fascinating, honestly. It's an entirely different world than the one I'm used to, from terminology to how systems are structured.