No other reason than "other than Microsoft made it so"? Marfig, I'm surprised at you.
DX10 was MS's opportunity to completely break the old DX models, which had been in existence since before the very concept of shader cores and about 90% of the other things that a graphics card has on it today. Windows Vista radically redefined the HAL, and was a good time to implement such a drastic change. Re-coding it to go back to the way XP interacted would hardly be a trivial undertaking and wasting that effort supporting a nearly 10 year old OS at that point simply because people didn't want to move would be a waste of resources.
You act like the move to Vista was a simple step from XP (Win7-Win8 is much, much more streamlined, and Vista-Win7 was as well - the HAL has not substantially changed) when in reality part of why it was such a bear was that it was truly a new design for MS from the ground up...they got a lot wrong, but the heart was in the right place, and Win7 was proof of that.
Forcing users to upgrade to win8 from win7 would require a product obsolescence that would put MS in the doghouse for a ridiculous timespan. It's true that many people will probably give this version a pass, but I'm tired of hearing people say "Look what they did with DX10 to try and sell Vista!" You're a programmer, you know better!!!!