There were two blog posts around the why:
Making it easier to port to .NET Core https://blogs.msdn.microsoft.com/dotnet/2016/05/27/making-it...
Changes to Project.json https://blogs.msdn.microsoft.com/dotnet/2016/05/23/changes-t...
Scott Hunter, Damain Edwards and Jon Galloway recently did a live community standup video about them https://youtu.be/YJsQ3tnS7Ew?t=10m39s