As far as I know (please correct me if I am wrong), then .NET Core should be open source and for Linux.

I am running Debian myself, which is known for having all open source software in the repo. But I cannot find .NET core?
Does anyone know why .NET has not been accepted into Debian yet? Is there something wrong with some of its dependence that stops it? Or is it because of quality? Or is it the lag of manpower from MS side that is holding it back?