There is an option in Tools -> Options that enables preview versions of the .NET Core SDK. In the VS Preview shipping channel, it is on (by default, and not settable). In the VS Release channel, it defaults to off and you can opt-in.
(Note: it’s disabled in the screenshot because I have a Preview build installed.)
The reason for this is to allow you to customize whether a project using a released version of .NET Core (such as 2.1, the current LTS) will use tooling from the newer not-yet-release-quality SDK.
Prior to 16.1, this option was located on the Tools -> Options -> Projects and Solutions -> .NET Core page.