The really tried to conquer the living room with the Xbox One, making it more about media than games, and the gamers revolted while the non-gamers shrugged. I think that's when the rest of Microsoft wrote it off.
There is no .NET Core 4 because people would get confused with .NET Framework 4 which is still getting bug fixes and has support. And they wanted people to move from .NET Framework 4 so they called it .NET 5 instead of .NET Core 5.