These two different approaches are microcosms of Apple and Microsoft’s very different approaches generally. Apple envisions what they want the future to be for their company (based both on what’s optimal for them and what they see as the best solution for users), and figures out how they’re going to get there from where they are now. This may mean sacrificing existing products, or radically changing them—everything is up for consideration. Microsoft, though, is responding to market trends (touch as a means of input) while attempting to protect their existing business (Windows and its existing applications).
The best writeup yet on the topic.