> It's shocking to me that Microsoft aren't heavily involved with the project considering it's one of the fundamental frameworks for releasing software on Windows.

The history of WiX is that it started internally at Microsoft. IIRC it was a project under the Office organization originally. It's generally considered the first big open source success of Microsoft in spinning a project out to open source community ownership and paved the way for almost every later open source project at Microsoft.

I've got a feeling Microsoft doesn't want to support it anymore because they see it as completely legacy today. WiX is one of the better/cheaper/harder ways to build an old school MSI file. MSI installers are an ancient archive format (the old CAB format) wrapping an ancient and dying DB format (the old JET database engine) and a lot of the complexity of the WiX toolkit is just a reflection of the complex legacy of the old terrible MSI output format. Today Microsoft suggests using MSIX which looks a lot more directly like the better/simpler input to a (well crafted greenfield) WiX project, it's a plain ZIP file with XML metadata.