oh wait. it's not like they did. they did say it.
Plus, vista was released in 2007, XP SP2 (the most popular version) was in 2004. so its like ~3 years diff. So its not like hardware has progressed in 3 years, its more like new software got significantly slower
They WAY overreached in their goals with Longhorn. When they finally decided to cut back features to something actually attainable, they didn't have enough time to make a high-performance OS.
Windows 7 was a well-loved rebrand of what was essentially just a Windows Vista service pack and improved performance (though it was still too heavy for a lot of the older machines people tried to upgrade to Vista). If they'd have cut back on their goals earlier, Windows 7 is likely a lot closer to what would have shipped as Vista.
Windows 7 benefited from coming later with Vista being the battleground in which vendors were forced to update to NT6.0 models.