When you have thousands of XP machines, many of which lack drivers for newer versions of Windows or the hardware necessary to run them well, and applications that haven't been updated for over a decade... well, you do this to keep yourself (somewhat) more secure while you work out the transition plan you've been putting off for far too long. (And Microsoft is only offering this to companies that commit to a transition plan, although it's unclear on what timeframe and with what if any penalties if the transition doesn't happen.)
Aren't Microsoft's solutions to running XP stuff on newer versions of Windows pretty good? XP Mode works well for our exotic software, but I have no idea how it scales for many users
Now that support has ended, Microsoft's recommendation is that you "only use Windows XP Mode if your PC is disconnected from the Internet". Source:
At some point you reach either enough users or an important-enough application where pretty good is not good enough, especially if you haven't tested it yet.
for some the reason not to upgrade to windows 8 isn't the license fee. It's the fact that they simply can't because their custom app doesn't work on windows 8 or they can't afford to spend IT time on migration/testing.