I think Microsoft coming out with XAML/WPF created too much complexity for most of the developers that were working with WinForms. Although the code had better separation from the interface, it also took quite a bit more effort to get an application up and running quickly. They took the "R" out of RAD, and then kind of put both WinForms and WPF on life support, while continuing to tout cross-platform on .NET Core without a first-party GUI platform. Now it just looks like Azure was always going to be the solution, to tie users into a platform they don't fully control.