The same market forces that rushed Win7 developments into Vista to save it after it was first released and was considered a resource hog disaster. I think they might have hoped that consumers would only upgrade hardware to meet Vista's needs but until SP1 it had a Typhoid Mary reputation - not remembered as such now because SP1 made it tolerable thanks to Win7 genius.
I also don't think we can ignore that business is migrating into Win7 and isn't even looking at Win8. Does that segment not represent a significant market force for keeping Win7 at the fore? Will more PC buyers demand Win7 since the smart people want it, and it's still wildly popular and only 3 years old?