Because people want DLC that can live up to the Shivering Isles from Oblivion. They want a whole new landmass to explore. Thankfully, Dragonborn does that pretty well. Without question the best Skyrim DLC so far.
Dawnguard was nominted because unlike Dragonborn, it actually came out before the nominations took place.
[Edited on 12.06.2012 8:05 PM PST]