.NET Core might be portable, however it is a tiny subset of what .NET Framework is capable of.
Additionally lots of enterprise shops, despite what HN crowd thinks, are mostly on Microsoft stacks, so that .NET application is going to connect to other APIs and stuff not available on GNU/Linux.
Then, many companies use VMs as desktops for contractors, you are just not allowed to plug anything on their network, so you are using Visual Studio via RDP/Citrix.
As an example, in life sciences most of the laboratory hardware only has APIs available via COM/.NET Framework.
Additionally lots of enterprise shops, despite what HN crowd thinks, are mostly on Microsoft stacks, so that .NET application is going to connect to other APIs and stuff not available on GNU/Linux.
Then, many companies use VMs as desktops for contractors, you are just not allowed to plug anything on their network, so you are using Visual Studio via RDP/Citrix.
As an example, in life sciences most of the laboratory hardware only has APIs available via COM/.NET Framework.