Windows XP was released back in October 2001, and still today, many users and businesses still use it every day. Even in 2012, XP's market share is around 27%. Is your business still using Windows XP? How much longer should XP be kept around?
Windows Vista put a bad taste in a lot of people's mouths, so for many, sticking with what worked was much more attractive than stepping into the unknown. Windows 7 vastly improved upon the issues found in Vista, but by then many users were content with keeping their older PCs alive.
Microsoft does their due diligence to ease the world into a new OS; they support older operating systems long after they are replaced and work with hardware manufacturers to make it easy to upgrade to the latest incarnation when users purchase new laptops and PCs. Of course, with stellar IT management and a little luck, PCs can last a lot longer than expected, so users don't always feel the need to upgrade.