Yes, they have to justify combining all six films in one package.
Because, really, they are a strange and unwieldy group of movies to stick together in one box set. No matter how long the prequels have been around, the trilogies still just don't seem to have anything to do with each other. The tone and level of sophistication are just so incongruous.
Airing this apologia is kind of necessary to create public perception that the movies are of like quality / aesthetically related. Necessary, but not sufficient.