What is there to lose? 0-Day attacks. Knowledge about bugs in aviation software is potentially more valuable to people who wish to do harm than to the people who would fix the bugs, so there's a concern that someone who finds a bug will sell that info rather than let the maintainers know about it.

The other problem is that the maintainers have to be set up to handle a potential avalanche of comments, criticisms, questions, and pull requests, mostly from people who don't know anything about software development processes and standards within the aviation community. If they're already too overloaded to find all of the bugs themselves, they certainly won't be able to effectively manage open-sourcing their code.