Related: Firefox, Chrome, Safari, and Chrome Frame are all more advanced than IE9 and all run on Windows XP.
And the reason for that is probably that Microsoft keeps so much backwards compatibility that the cost of not upgrading the OS is much lower than on OSX.
Or they need people to be paying for commercial support of existing products. I'm not privy to any sensitive details, but I'm guessing they make the bulk of their real money from businesses, and that most large/profitable business customers have some sort of ongoing/bulk deal with Microsoft rather than paying for licences individually. Curiously, those large/profitable business customers are probably also the ones who don't like things like pushing updates every few weeks, because they (perfectly reasonably, IMHO) want a controlled software installation on their standard staff PC and they want to test any changes in-house before committing to adopting them.
It's not within Microsoft's strategy of planned obsolescence. For their continued survival they need people buying newer versions of the same software.