Vi and Emacs are older projects with a larger community, and thus there's a ton of plugins out there even for niche applications. Xi is much newer, and attempts to solve many of the problems that Vi and Emacs face (examples: in Vi's case the difficulty of writing extensions, and in Emacs' the difficulty in maintaining and extending the core of the editor written in C). Xi also focuses in areas that weren't considered that important when Emacs/Vim were first developed, such as asynchronous operations and collaborative editing.