It's all about the long term strategy
Microsoft only make money with Windows and Office, everything else they do bleeds cash, and they know this isn't going to last forever. So what can they do? Copy Apple! Take a fee for every app / program sold on their OS, that's how.
So, they have developed Metro as the successor OS to Windows desktop, where commercial apps are only available from the Microsoft Store (Enterprise apps can be distributed directly, as per iOS) and a large proportion of the revenue goes to Microsoft.
Windows 8 is the first step in this direction with the addition of Metro to desktop, but as Windows Blue arrives and we get annual updates to the desktop OS with relentless promotion of Metro apps we will see depreciation of desktop by stealth (like we did the transition from DOS to Windows in the past). They've given up on backwards compatibility, as few care (those that do can run VMs) and it doesn't help the cash flow.
Windows RT was obviously perceived as a driver of Metro apps, but really it's the timing that is wrong. It may be DOA but it will be like a zombie and come back from the dead because eventually Metro will be the mainstream Windows OS and desktop will be as relevant as DOS is today.