I think VS Code was supposed to be a "gateway drug" to VS. It's not turning out that way. If they can't get the users to go full VS, they probably have an idea they can sell some of the backend stuff, but they can't right now since there is not a way to plug it in. OmniSharp is probably going to remain the default, but MS is betting a number of people will pony up for better(their) backends.