It's not so much that it's "hard", but that it's time consuming when you have hundreds or even thousands of python scripts to port -- and since those scripts already work and you probably weren't going to have to touch them at all, you're not really gaining anything for all of that porting effort.
If you'd have been writing python a decade ago, you'd know why people couldn't transition immediately to Python 3 even if they wanted to. I no longer work for the company that has hundreds of Python scripts left to migrate, but I don't think all of libraries needed (including some API libraries from vendors) were ported to python3 until a few years ago.
