I contacted the owners of the Microsoft.CrmSdk.CoreAssemblies NuGet package today and got this response:
We support .net core and .net standard clients connecting to and utilizing our web API.
We are evaluating .net core/standard for the future, However we have no short term plan to move over to .net Core\ .net standard for our SDK Libs.
The truth of it is that 90%+ of our SDK logins are based on WCF/WS-TRUST and UserPassword Based flows.
The teams have provided part of the WCF Support we requested, however WS-Trust has not been adopted going forward in the .net core / standard libs, nor has the UserPasswordFlows that the majority of our population use via connection strings and direct logins.
I expect that your teams ask Is based on being able to use the object model and the patterns that we support in the .net SDK and possible the authentication and logging support we provide in it.
We are working though it with the support of various teams.
Update 1
A preview of the .NET Core assemblies is now available. See Announcing the .net Core SDK for Common Data Service (CDS) - External Client Development.