Microsoft Explains Why It Decided to Patch Windows XP Despite End of Support
http://news.softpedia.com/news/Microsoft-Explains-Why-It-Decided-to-Patch-Windows-XP-Despite-End-of-Support-440318.shtml
That was my best guess, XP with some 30% of OS Market share (not sure about their 26% claim) and many of then still using IE - if MS didn't patch then many of them would be forced to migrate to other browsers. That would be a big drop in Browser market share for IE.
Once they migrate to another browser, who knows even if they upgrade their OS they may stick with their new browser. MS want you to stick with IE, but to upgrade your OS, unfortunately for many that would mean a system upgrade/replacement.