I'm disappointed it doesn't explain why IBM stopped working on it.
I bet the question was posed, but the answer was "no comment".
In any case, it doesn't really matter. Obviously, they think the manpower is put to better use elsewhere.
They sort of do answer it here:
"Swift benefits greatly from being an embedded part of Apple's ecosystem, but it also provides challenges for its organic growth as there's a lot of dependency on Apple for capabilities that the server ecosystem requires. For example, almost all Swift developers use Apple's Xcode as their IDE and it has fantastic support for developing for iOS devices, including the ability to run locally in emulator environments. It would be great to add support for writing server code that allows developers to develop directly into containerized environments from their local IDE by supporting the simple integration into Xcode of development tools such as Appsody. Where there is open governance and open ecosystems, it enables the community to contribute and to solve the problems and use cases that are important to them."