That's because Windows has to deal with so many more differences in hardware. E.g. look at these pixel-level variations in LCD screens. http://www.digitalversus.com/tv-television/screen-technology... I don't understand how it would be possible to support subpixel rendering on some of those. IIRC when I last used Windows (XP?) it let you choose from 8 different schemes. But that still wouldn't nearly be enough!

OS X and IOS can be tweaked to support a much smaller subset of that mess.