I've been waiting since New Vegas first came out for this. I purchased some of the Fallout 3 DLC twice since I purchased some individually then picked up the GOTY edition later on and didn't want to do that again.
I think you are right, but I wouldn't be surprised if it's also because they don't want to compete with their own game. Sales will most likely be higher once Skyrim settles done and people are ready for another RPG. Since so many great games come out each month now I always wait for collections like this and will be patiently waiting for the Skyrim Ultimate Edition next :)