The simple answer is not having to maintain a really complicated language bridge anymore.
The more complex answer is that Hammerspoon is currently about 100k lines of Objective C, and none of us really want to work on it anymore when Swift is the much nicer place to be doing macOS development.
Technically we could slowly convert in-place from ObjC to Swift, but there will always be a need for "LuaSkin", the bridging code we've accumulated over the last 13+ years, and rewriting that in Swift would be significantly complicated.
JavaScript, however, is already bridged for us because WebKit needs it.
The more complex answer is that Hammerspoon is currently about 100k lines of Objective C, and none of us really want to work on it anymore when Swift is the much nicer place to be doing macOS development.
Technically we could slowly convert in-place from ObjC to Swift, but there will always be a need for "LuaSkin", the bridging code we've accumulated over the last 13+ years, and rewriting that in Swift would be significantly complicated.
JavaScript, however, is already bridged for us because WebKit needs it.