There's actually a very good reason. Windows XP has been officially unsupported by Microsoft for about two years now unless you are willing to pay them a quarter million dollars a year. Often the computers spec'd for those systems have pitiful amounts of CPU power and memory (i.e. much less than necessary to run Windows 10), and the cost of a re-spec for PCs in certain critical systems is basically the cost of developing a new platform (not to mention that some of them can't fit a microATX-sized PC or have low power requirements) makes it potentially cost-effective to use SBCs, especially a popular one which someone like Newark/Element14 can customize for you and potentially support.