Microsoft Is Being Smart
If you've written WPF programs, you probably saw how slow they were! i.e. I had an 8 core i7 box and the 3D graphs, from Telerik, were pokey! So 3D WPF graphics suck!
And that highlights the inherent problems of WPF and Silverlight: they don't enable performant applications!
For example, if you look at C#, it has to stop all the threads in the App Domain when it garbage collects! That issue was supposedly worked on in .NET 4 but, again, .NET is, for the most part, a slow platform. For example, when I did database timing, .NET was 10 to 20 percent slower! And, when we implemented MVVM applications, my team had to use memory profilers to eliminate hundreds of .NET memory leaks since .NET bindings, etc..., weren't giving up their references.
At the end of the day, HTML5 is where it's at since the average person doesn't have the time and money to write performant Silverlight or WPF apps, it's simply not possible. And, If you doubt that statement, look at what Charles Petzolds talks about since, essentially, he proved that the MVVM model doesn't scale; thus, from those comments, I concluded that XAML fragments memory and code too much since it encourages a million tiny objects and a million non-contiguous ( like lambda expressions ) pieces of code.
IMO, Microsoft seems to have reached this same conclusion:
"IE10 continues on IE9’s path, directly using what Windows provides and avoiding abstractions, layers, and libraries that slow down your site and your experience"
i.e. WPF and Silverlight took abstractions and layers to their extreme and to the point where good performance was impossible or, put another way, to the point where the web browser's UI engine left WPF and Silverlight in the dust.
Besides those issues, Silverlight doesn't interoperate with host apps that well; i.e WPF and Silverlight don't support vector graphics at their core! So you can't copy and paste Silverlight content into other content "out of the box;" That's why the move to HTML5 has to happen since browsers are going to support "rich content" copy and paste.
In closing, I wish that .NET didn't create more problems than it solved but, these days, I wonder.... ;-)