Naming is hard.
There are only two hard things in Computer Science: cache invalidation and naming things. - Phil Karlton
It's very easy to armchair quarterback and say that "they should have named it Foo and it would be easy" but very often there's many players involved in naming things. ASP.NET is a good 'brand' that's been around for 15 years or so. ASP.NET 4.6 is a supported and released product that you can get and use now fromhttp://get.asp.net.
UPDATE NOTE: This blog post is announcing this change. It's not done or released yet. As of the date/time of this writing, this work is just starting. It will be ongoing over the next few months.
However, naming the new, completely written from scratch ASP.NET framework "ASP.NET 5" was a bad idea for a one major reason: 5 > 4.6 makes it seem like ASP.NET 5 is bigger, better, and replaces ASP.NET 4.6. Not so.
So we're changing the name and picking a better version number.
REINTRODUCING ASP.NET CORE 1.0 AND .NET CORE 1.0
ASP.NET 5 is now ASP.NET Core 1.0.
.NET Core 5 is now .NET Core 1.0.
Entity Framework 7 is now Entity Framework Core 1.0 or EF Core 1.0 colloquially.
Why 1.0? Because these are new. The whole .NET Core concept is new. The .NET Core 1.0 CLI is very new. Not only that, but .NET Core isn't as complete as the full .NET Framework 4.6. We're still exploring server-side graphics libraries. We're still exploring gaps between ASP.NET 4.6 and ASP.NET Core 1.0.
WHICH TO CHOOSE?
To be clear, ASP.NET 4.6 is the more mature platform. It's battle-tested and released and available today. ASP.NET Core 1.0 is a 1.0 release that includes Web API and MVC but doesn't yet have SignalR or Web Pages. It doesn't yet support VB or F#. It will have these subsystems some day but not today.
We don't want anyone to think that ASP.NET Core 1.0 is the finish line. It's a new beginning and a fork in the road, but ASP.NET 4.6 continues on, released and fully supported. There's lots of great stuff coming, stay tuned!