Because it's not unrelated.
Silverlight is deprecated for use in the browser, but lives on as one of the key technologies that you can use to write metro apps. Consequently, the Silverlight team have stopped blogging about Silverlight for the web (because it's deprecated) and focus on talking about Silverlight for Metro (since that's what they've been working on).
Silverlight to me was WPF/E, and the promise of a cross platform client was what it was all about. Microsoft seem to be trying to neutralize dissatisfaction with saying "It's .XAML, so its the same". Silverlight was quite different to WPF, even in the .XAML, with commands missing and things like the image libraries behaving completely differently.
Windows 8 is not about building web applications, which is what Silverlight was, so associating it with Metro is completely wrong, and Microsoft desperately trying to soften the blow for the thousands, if not millions of developers now left with brand new, but obsolescent code.
I would also like to add that it is tremendously difficult to get software to the point that it is stable, and even migrating .XAML platforms is fraught with difficulties, it is not as easy as flicking a switch, especially since you are already seriously narked off that you chose a Microsoft platform that was abandoned.
Do Microsoft really think their developer base is that idiotic, where you write a WPF app, port it to Silverlight, then Windows Phone 7, then after killing off all these platforms, they think you are eager to migrate to Windows 8 or Windows Phone 8, or even if you did nothing with them, will somehow start getting on the Metro bandwagon?