Microsoft is desperately trying to avoid the fate they inflicted on the Mac in the 1990s with Windows 9X:

Office for iPad, launched at the same time as Windows 8/RT, would most likely have killed the market for Windows 8 and RT devices. As it was, that market was already severely diminished and below expectations. But with a viable alternative tablet, it could have been game over. And the ramifications of that decision would have impacted far more than just Windows 8/RT: The PC market could have literally collapsed, much as the video game market did in 1983. The fallout would have included PC makers going out of business/being sold, a serious and potentially permanent hit to Microsoft’s bottom line and the ouster of Steve Ballmer. I’m talking tech Armageddon here.

via A Theory about the Office on iPad Schedule | Office 2013 content from Paul Thurrott’s SuperSite for Windows