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Money is probably the biggest reason, though there may be others including inertia. If I were going to dump Windows XP, I'd go with Linux. There are tons of Linux distros that would work well as alternatives to Windows XP.
See DistroWatch for information about all kinds of Linux distributions.
Money would be one big thing. Keep in mind, at many companies the people controlling the purse strings aren’t really tech oriented. So if it still works, why spend money on an “unnecessary” upgrade? They aren’t really aware of the security implications. That’s what IT is there for, in their book.
I also know a few people who are very averse to change. In fact I know someone who brought her personal laptop to work, and it was running XP. I looked it over, and it didn’t even have Service Pack 2. I pointed out that not only was the OS obsolete, it was not secure with a cherry on top because it had never been patched or updated. She still refused to update it because, “It’s working fine.”
Also, you have some machines that are used for very specific purposes. I know of one XP machine that is used to control machinery. It isn’t on a network, it does the job fine, and in situations like that, there really is little compelling reason to upgrade.