If XP were still in the lead it would, most likely, be due to hardware limitiations. There are still many people who cannot afford new hardware that will run modern OSs. There are also many people who are afraid to go on to something they are unfamiliar with.
There would, probably, be more people still running 98SE except it has no support for modern internet speeds and content. There are also many small businesses who either, cannot afford to, or don't wish to lose productivity and are content with sitting on XP for now. The county was running Win2K up until a few years ago (2006/07) when they finally upgraded to XP on most workstations. The recent push to Windows 7 came from our Help Desk... Which is one thing that I commend them for (getting county council to spend money is like trying to wring water out of desert sand)...
Many hardware upgrades were made recently and there was one workstation in my building running Windows 2000 Pro up until a few weeks ago.
XP isn't dead yet... but it is on it's way out of the mainstream. It won't be long before XP goes the way of 98SE... I am sure there will be a cult following for many years to come! But it is time to say goodbye.