Because older versions of the demos were too old, they were just prototypes that are no longer representative of final product, thus no longer serve the purpose of a demo.
They are not even remotely close to how finished game will look like, so to avoid confusion they were removed.
To allow easy modding the game needs to be specifically designed for it which makes it more difficult for developer. I have a certain vision in mind and I want the game to stand strong on it's own. If this was a sandbox game then not having mods would be an issue, but story based games are totally fine without modding support.