A better question is why anyone would want to keep using OS X 10.10 (the worst version of OS X ever made outside of Lion). It got way uglier and 4x slower. Nothing else to see there. Frankly, I'm still wondering why I bothered going to 10.11 when I have yet to see a single game support Metal and the GUI is only faster in the sense that it undid the damage of 10.10 (XBench reports way slower traditional scores).
I would actually love to upgrade but sometimes software limitations keep people stuck. For example, for over half a year Avid Media Conposer (which I use for work) wasn't compatible with 10.11 so I couldn't upgrade. I can now but need the money to replace my dying hard drive before it will let me.
Point is, there are often limitations aside from want that hold people back.