One of the interesting questions around why so many organizations hadn’t migrated from Windows Server 2003 by the time the end of support date rolled around was “what on earth made people stick with a 12 year old server operating system so long?”
The simplest answer? It did what people needed it to do. Migrations are challenging and difficult and it turns out that just because a new version of something is released, people don’t feel compelled to move to it. If the server does what you need it to do, it’s not as though moving to a new version of the operating system is going to make it do what you need it to do “even better”. At best it will give you more options or do what you want it to do. Sometimes though, a tool does the job that you want it to do. Even when “better” tools come around, there’s no real impetus to abandon the original tool.