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Visual Studio Code is an entirely different animal, built with JavaScript on top of node.js (like Atom). MonoDevelop is a clone of Visual Studio written in C# which isn't even remotely compatible with the mis-named Visual Studio Code. If they want to port MonoDevelop to Visual Studio Code, they would have to re-write all the features from scratch. At this point they might as well re-write Visual Studio to node.js and…
OmniSharp powered intellisense and other features are easy enough to stick in VS Code. It's easier to package and deploy than MonoDevelop. Much easier (at least for me) to write extensions for VS Code. I see it as a win-win. MonoDevelop has always been a frustrating experience for me. While VS Code isn't perfect, I'm less frustrated using it than MonoDevelop.
Already shipping with it. See: