.NET Core is available on Linux, FreeBSD, and Mac, so you can use it for servers on Linux.[1] A lot of developers (such as myself) use Macs for their development machines, so it's very useful to have the same framework/runtime for both Mac and Linux.

[1] https://github.com/dotnet/coreclr#get-net-core

Because their tech stack is pretty good from developer experience point of view. Good in the sense developer speed is good, as in speed in developing new features, understanding the code and debugging. There are not "tons" of good industrial strength free/opensource alternatives to .net stack. There are, but I've not seen one that would be of better quality. In ways Visual Studio is the gold standard of developer experience (unless one goes shell and makefiles all the way but that's... a different demographic I suppose).