Aren't those GUI frameworks like Qt programmed in C++? There is difference between using scripting language for easier work in GUI designer, for styles or whatever while you're still coding in C++ in relatively monolithic structure (but yes, it's already falling apart in contrast of FPC/Lazarus) vs using C# which means second language with different concept. I completely do not understand what has prevented large companies at least from keeping one way.
Of the big cross plattform frameworks, QT is written in C++, mainly for C++ (and therefore requires C++ to C wrappers for using in other languages), and GTK as the name (gimp toolkit) was written for gimp, which is a C program, so GTK is written in C.
These have especially been established because you can use them with any other language (for example you can even use GTK with C#). Then there are the platform dependend Frameworks like .Not with .Not Windows Forms and WPF, Java with JavaFx and Apple with Cocoa (Android sure has it's own thing, but I don't know the name about that).
But what these libraries are written in doesn't matter much regarding whats their best usage. .Not is surely internally written in C++, but still C++ is pretty much the worst language to use it. That said, there is QT with the QT creator for C++, which comes pretty close to the type of RAD development we know from Lazarus. You have QTCreator as your IDE, with a pretty WYSIWYG editor, the event system is pretty weird (basically the event function is defined by name, so button1 will always call button1_on_click or something like that, which is really weird), but there are these tools. But as I said, this is not the C++ main niche. And without any demand there is not much development.
One of the main things about C++ is, it isn't and never will be a Microsoft language.
Microsoft directly develops C#, they have full control there. It's better? Let's make UI in eeee.. Net.. uhm.... stand.. ee.. .Net Framework? Or newly and partly in .Net Core (which is different from Framework)? Or maybe Mono, UWP, WPF, WTF, Xamarin, Silverlight, ASP.NET/Core/CoreMVC/.. webforms, Entity Framework.. or wait until november there will be new .Net 5.0 which "takes best from all this" except half of the concepts will be forgotten and or implemented again in semi-finished phase at least until new direction. There can be no question that the user just simply starts Visual Studio and just makes an GUI application.
.Not is basically pretty simple, it started out as the idea: "Lets make Java but better", then the demand for cross platform availability increased (which was first met by mono, which is an open source clone of .Not and had at first nothing to do with MS) they decided to also support this, but as many of .Not features where directly representative of WinAPI features, they couldn't just port .Not, and rather than that put out .Not Core.
WPF was developed in an effort to make "pretty" application development easy by basically having DirectX rendered forms (similar to firemonkey for delphi, even though the reasoning behind it is different) which allows for much more complex graphics. Xamarin is an IDE containing a cross plattform gui framework they bought (before that it was a totally separate project, so not the idea of MS) which now basically is the Mobile version of .Not, ASP.Net is a language, MVC is the micrsoft LibC and entity framework is a framework for Databases. And Silverlight was a mistake.
I mean sure they did a lot of stuff, but Microsoft is a large company, and most of it (except for Silverlight) was a success. I mean if you want to use C# you simply download visual Studio and can start of. You want to have more graphical opportunities (which makes it also more complicated), Simply use WPF. If you want to make an (cross plattform) application that does not requires system API specifics (like GUIs), use .Not Core, if you need a web application, use ASP.Net. You want all of .Not but cross plattform and maybe not by MS, use Mono. You want to create a mobile app, use Xamarin.
Btw, I really don't like .Not, and haven't been following it since 2014 or so, but still I know and understand their Concepts and products
I mean they made it such that a formerly Windows only Java Clone, is now one of the most important Programming languages around. Meanwhile VC++ is still only considered "just another C++ compiler"