The reason for these changes dates back to the end of 2014, when Microsoft forked the .NET Framework to create .NET Core. At that time, Microsoft announced it planned to open source more of its .NET programming framework and take it cross-platform to Linux and Mac in the form of ASP.NET 5, its server-side web-development framework. The centerpiece of that effort was .NET Core -- a fork of the .NET Framework.
The current ASP.NET 4.6 implementation isn't based on .NET Core; it's based on .NET Framework. So calling the coming, .NET Core version of ASP.NET was confusing, as it implied it was the successor to ASP.NET 5, rather than the start of a new ASP.NET family.
While a lot of Microsoft's future focus is on .NET Core, officials said back in 2014 that the company planned to continue to update .NET Framework about once a year, with some subset of features it introduced in .NET Core. There also will be some features added that will be exclusively for .NET Framework, like some of the features Microsoft will be adding to support the Windows Presentation Framework (WPF), execs said.
Update (January 26): Microsoft just updated the roadmap page. Release Candidate 2 of ASP.NET Core 1.0 is now listed as "TBD" (to be determined). It had been February 2016 until today. And the 1.0 RTM date is also now TBD in 2016 instead of Q1 2016.
"We thought that by the end of January 2016 we would have DNX (.NET Execution Environment) features and experiences implemented within and on top of NET Core and the new CLI. We realized, along with other folks on GitHub and Twitter that we don't have all those bits working together yet and we knew that we would miss the date."
The intent is to let the changes bake a bit more, the post says