Jetbrains was using a package provided by Microsoft and blogged that they would soon have to disable debugging of CoreCLR

Read “The Bad News” https://blog.jetbrains.com/dotnet/2017/02/15/rider-eap-17-nu...

Also the licensing issue https://github.com/dotnet/core/issues/505

Then a week later with the licensing issue still in place Jetbrains implemented their own debugger, on the comments it reads "This is our own debugger implementation." https://blog.jetbrains.com/dotnet/2017/02/23/rider-eap-18-co...