> The Roslyn sdk is part of .NET (cross platform) these days so anyone can use it to build a visualiser should they wish to.
Of course, but for most people that need this, they may just be trying to figure out how to do something as simple as adding an analyzer rule to a project, and don't have the luxury to be able to delay that work to spend weeks cobbling together a visualizer using an SDK that they may only have barely any knowledge with.
There are certain things that don't inspire the kind of passion it takes for volunteers to do this, and therefore it's up to those who have a financial motive to do so. Whether it be the rent-seekers at JetBrains, or someone other than Microsoft and doesn't have an incentive to make a competing OS more viable for developers. Without the .NET foundation's governance model and funding (regardless of who funded it), it would have never made it this far. See: https://github.com/dotnet/vscode-csharp/issues/5276