Bryan Cantrill gave a talk that included how difficult it was to take something that wasn't originally intended to be open source and then open source it. I think it was something at Sun before they were acquired by Oracle (iirc Cantrill laments how Oracle effectively un-open-sourced the project). I've watched hours of Cantrill on youtube, I'm having trouble finding which video it was. I can't explain it myself but I remember my takeaway was "wow that's a lot more complicated than I thought."

Edit: Child comment has the video. Starting at 20:17, "We wanted to open source the system. So why didn't we? What was the problem? Well, the problem was that it's a deeply proprietary system."

Actually, it very much is a valid reason.

The hubris of those who believe other people should be forced to release their (not already subject to a copyleft license) work is the very antithesis of FOSS principles.