One simple reason would be that an undo tree is unusable without a way to visualize it, which is exactly what the various Vim and emacs plugins provide, and not

One simple reason would be that an undo tree is unusable without a way to visualize it, which is exactly what the various Vim and emacs plugins provide, and not that useful in practice anyway. So you have to consider the cost of developing the underlying mechanism _and_ the UI and the benefits of working on a niche feature.