While Silverlight did produce the first portable version of .NET and produces a lot of the code that allowed .NET to be portable, this code has been now retrofitted into the main CoreCLR and the VM source code has been upgraded.
What you see in the public CoreCLR is now the state of the art and while I do not know how the exact mechanics work, my understanding is that the CoreCLR and the .NET Desktop VM that ships source codes are very close to each other - modulo branches, deliverables, freeze dates and other loose ends.
On the library front, there has been a lot of rewriting for the sake of portability, cleaning up the code, and making the code more maintainable, and bringing it to the 2017 standards of coding.
And it's not a matter of priority or security, it's a matter of transparency. The community was promised a review, and stress test, and that someone from MS would be working on it full time. They followed up with more promises multiple times. And then silence. You're completely missing the point.
.NET Core has been a huge rewrite of the old framework, focusing on common scenarios which are easily cross-platform first, then the harder ones later. E.g. System.Drawing, which they eventually included (in 2.0 in believe) via incorporating several implementations for each target architecture, including one from Mono.
They'll probably get to AppDomains eventually, assuming it isnt outright impossible on linux/osx or introduces a massive security hole.
But not doing it right now cause you want it isnt evil: which is more important? Full support for System.Crytography, or say Oracle Ado.NET, or runtime loading/unloading of dlls? Hint, the answer isnt dll unloading.