Re: Why can't they
Because all the market growth is in mobile, and Microsoft doesn't have a competitive product there. How many people do you know who've bought a desktop PC for their home recently, or even a laptop? And how many businesses are adopting Windows 8?
The number of people recently still on XP is indicative of a long-term slowdown in the Windows upgrade market. MSFT is trying to catch up in mobile by leveraging their traditional presence, but people have got used to Android and iOS. MSFT's traditional partner, Intel, is in a similar position.
It's got to the point that you can hook up a phone to a keyboard and TV to make a "PC" (non-IBM-compatible) that is quite usable for browsing, social networking or working on your cloudy stuff, which are apparently what people are interested in these days.