.......In order to get the most from their video cards, users would have to upgrade their operating systems to Vista.
Given XP shipped in 2001 and it was late 2003 when the DX10 design solidified - it should be obvious that 'what the OS was' was well beyond XP before serious DX10 work commenced. Heck, the Longhorn reset was in 2004 and DX10 wasn't done until later. The build that was demo'ed [sic] at WinHEC 2004 with the texture memory management was a very fresh build and wasn't feature complete - and that was April or May 2004. The 1st DX SDK supporting DX10 didn't appear until Dec 2005.
Taylor also noted that it would take a tremendous amount of work if the company were to retrofit DX10 into XP.