It's complicated, but the short answer is that the Windows team messed up and there isn't really a good new native UI framework to migrate Regedit to.

UWP was burdened with sandboxing+packaging requirements that made it very difficult to use, even for first-party software. They eventually figured that out and have made some attempts to lift UI stuff out of UWP (XAML Islands and WinUI 3), but they're both train wrecks (and not staffed with nearly enough people to put up a fight against Chromium).