Good lord, in this day and age
While there may be valid reasons to jump to a newer OS, individuals often prefer that with which they are familiar. As for "this day and age", the IS department at my place of employment still has most of the office computers on Windows XP, simply because a LOT of the custom business software that the company paid millions of dollars for several years ago simply will not work properly a newer OS. Heck, we are even still stuck with IE6 because someone thought it was a good idea for a major database's front-end UI to be written such that it runs in a web browser, and will not function correctly in anything other than IE6. It's very nasty state that will be extremely expensive and time-consuming for the company to get out of. Its just one example of a real-world reason why an upgrade may be deemed undesirable, but I'm sure most people can think of many other reasons too.