Doing stuff in XAML is insanely easier than in the brain* that is HTML/CSS/JS.
With XAML they can continue to have full control over all aspects of the language and make sure that it keeps up with the current needs of the platform. Anybody who complains about XAML has probably never used binding. Try that in a browser without crazy JS libraries and other complications. With HTML, they are subject to whatever the standards bodies are smoking.