Mike Elgan at Cult of Mac explaining that Apple has transformed it's business to the point where PC's are no longer it's main source of revenue, as it once was with the Mac:
Microsoft doesn’t have this luxury. The company is in the position that no company wants to find itself in: It’s got to cannibalize it’s cash cow businesses in order to compete in the future.
In order to have a shot at dominating the post-PC future like it dominates the PC present, Microsoft would have had to do what Apple did: Aggressively build an alternative to the biggest and most important product lines.
The whole article is a good read and points out one of the key differences between Apple and Microsoft.
Apple is always thinking ahead, innovating, and not afraid to cannibalise it's own products (think what the iPhone did to the iPod, and what the iPad is arguably doing to the Mac), while Microsoft is holding on to the past trying to squeeze as much as possible from Windows and Office.