Well, for a couple of reasons, but none that any one voice in Redmond is yet including - and likely, not permitted - in a cohesive presentation.  First, take all of the mighty powerful things that are Silverlight and set them off to one side.  Next, stop to truly consider a couple of things.  To start, the essence of Silverlight is a minimalistic and entirely portable runtime that kicks the ass of WPF on the desktop, has the power to go head-to-head with Flash and makes anything in HTML4/5 look as entirely primitive as it is (and as will be the case for years and years to come).  Next up, that runtime's been proven viable on a Windws machine, a Mac, Windows Phone and the XBox, plus its been proven to work on jail-broken iPhones and other devices.  Going further, in what areas of Silverlight have we see massive investment in versions 4 and 5?  OOB (out-of-browser) capabilities.  All of this hopefully begins to suggest what Redmond is holding back - that Silverlight running as a browser plug-in is, to them, only incidental and of ever-diminishing strategic value.  Why?  Try this perspective, starting with why, when devices of ever-increasing power and decreasing size are bevcoming a commodity, why render anything at all on the server, really? Next, despite the fact that nearly all web applications that don't use Flash or Silverlight suck and that browser compatibility issues are a complete waste of time and money, what did web applications have to offer that made them worthwhile?  Ease of deployment - push your latest version to the web server or web farm and bang, everyone's running your latest and greatest version.  Now, two things have changed, two things that make all the difference: 1) The move toward mobile has made abundantly clear that users vastly prefer native applications over their lower class, low-budget counterparts and 2) the app store model, a key component in Apple's world dominance, makes it entirely possible - and worthwhile - to bring to market vastly superior, cloud-enabled native applications with just the ease of deployment that used to be exclusive to the web.  With the traditional web fading in importance and the major technology vendors having little or nothing to gain from the fallacy of 'write-once-deploy-anywhere', what would you do?  Pull back from Silverlight in the browser, pull back from Silverlight as a cross-platform vehicle, because neither has the longer term significance once imagined.  From that perspective, realize that WinRT is, heart and soul, the same - but perhaps evolving - Silverlight runtime, but a rose by any other name, and what it remains is a runtime capable of spanning Windows 8 on the desktop, tablet, XBox, phone and more.  Throw in the deployment of both public and private app store capability and what do you have?  A mighty, mighty ecosystem which, if not entirely mishandled, puts you in a place to actually compete in this vastly different world.  Silverlight as an HTML5 producer?  Nope, not needed.  Despite the lip service being paid to HTML5, it was never about that, not for Apple and not for Microsoft.  That it will continue to suck for web developers foolishly choosing to or stuck in developing in accordance with 'web standards' serves them, and many others, only too well.  Go native and go now...