I have asked a few developers who produced software they no longer maintain or let die, why they don't open source the software and let other developers take it forward and enhance it. Or at least not let it disappear.
Most of the excuses were about the quality of the code. They don't want to open source because they are ashamed of what they have coded. In my opinion, if the software works, people are downloading it and using it, that says enough about the software. No one is proud of their own code. If anyone looks at their year old code, you surely decide you can do better now. I have used some good software no longer exists. I find it this to be a waste of good resources.
Why would some developers prefer to let their software disappear instead of giving it to the public? One reason I can think of is that they used someone else's IP and they don't want to be exposed. Should developers be encouraged to open source their software if they have no intentions of profiting from it now or in the future?