davidjfelix outlines some of the issues, but I wanted to add in some that may have been simple upfront headaches that made people resistant:

- Simply put, print was ` print "text" ` instead of ` print("text") `. Looking back, this was such an annoying habit to break; but if you're codebase had thousands of print statements, that becomes thousands of changes

- range(3) used to return a list [0, 1, 2] while Python 3's range(3) returns an object. If your codebase relied on that explicitly created list, then it'd break in Python 3.

- Division was originally integer division, so again if you expected an integer and are now getting a float (with a non-numeric decimal point), then more crashes

- except used to be except (Error1, Error2) as e and Python 3 explicitly requires each except to be on their own exception catch

All in all, tons of changes that you'd need to do before switching otherwise your codebase would crash. It also meant you couldn't rely on Linux's default Python (2.7). I never needed to make the switch on a production base, but hopefully you can see why someone would drag their feet

[edit] Python was also still a "new kid on the block" of languages. Its popularity was growing, but since it was not an industry standard yet, these systems were mostly through hobbyist, so I imagine there was plenty of just trying to find that mythical "free time" I keep hearing about.