Just hypothesizing but I am guessing its easier to do this add-on on PS3 because its a separate map with its own set of items, locations, characters etc to keep track of rather than dropping extra dungeons and castles into the existing world map. However, you can freely travel between the two zones taking those objects and ongoing quests with you so I imagine all the same sort of things need to be tracked that would need to be tracked for Dawnguard... If they can do Dragonborn why couldn't they do Dawnguard? Is it something to do with the vampire systems?!
"if you dont like it do one elsewhere. stop getting up in arms as someone doesnt agree with you." - svd_grasshopper