Microsoft has been a juggernaut for the past decade because of the enterprise. Part of the reason for this are technologies like Active Directory and Exchange built on top on the Windows stack. Another big part of it are enterprise applications that are also built on top of that stack.
Windows RT (aka Windows on ARM) doesn't support AD management. It's instead employing a self-service style portal that's meant to address bring-your-own-device situations rather than corporate device management.
Additionally, Windows RT will only run applications written targeting Windows Runtime; traditional W32 apps aren't supported.
So here's the thing: from a corporate IT perspective, what does Windows 8 give me that other devices like iPads or Android tablets don't if I'm going to give up full device management, especially if I'm trying to address the BYOD problem? (And from a corporate IT perspective, wanting to run line of business apps on iPads etc is probably the most frequent request I hear from end users).
From an ISV perspective, if I'm rewriting my app to target a new platform anyway, why not just go ahead and move as much as you can server side, present the UI as a webapp, and then be able to support all these non-Windows devices that users are clamoring for anyway?
W8 really feels like Microsoft painting themselves into a corner by chasing the consumer market while doing a poor job of addressing the market that's made the company what it is to date.