When Microsoft officially announced that DirectX 10 (DX10) would only be available for Windows Vista, many gaming fans yearning to be on the bleeding edge were upset. In order to get the most from their video cards, users would have to upgrade their operating systems to Vista. Some have attributed Microsoft's decision to be purely based on marketing, but that's not entirely the case. What other factors were in play?
According to Microsoft DirectX guru Phil Taylor, development for DX10 wasn't complete until late in Windows XP's lifecycle, and during the time of its development, things became clear that DX10 simply would not fit into XP.
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. "Given the new features in the driver model and hardware (with GPU task switching, GPU memory management and more) all of which require kernel support - hoisting a driver layer like that on XP is rewriting it to be Vista," he wrote on his blog.
Technical reasons aside, money was also a big contributor in the decision to make DX10 exclusive to Vista. Microsoft felt as though Windows XP customers had received, as Taylor puts it, "good value for their money." Taylor even goes as far as admitting that Microsoft knew that making DX10 exclusive to Vista would certainly bring in new customers. "At some point, the question 'to serve existing customers' or 'to get new customers' is a question every business has to ask itself."
Overall, the reasons presented seem logical. It's clear that Microsoft would need to do some major work in Windows XP for DX10 to run just as smoothly as it does on Vista, and even the idea of luring new customers to Vista with DX10 isn't so bad. After all, many people have asked why they should upgrade to the new operating system. There are a variety of reasons, and being able to run DX10 is a bigger one. Would you agree?
http://arstechnica.com/journals/microsoft.ars/2007/2/14/7060