^Yes but the Avengers had everything laid out in the previous movies so all that back story was taken care of. For the JLA we are jumping into the pot while the fire is roaring. I'm not saying it couldnt be done but it would be a whole lot to squeeze into a single picture.
As has been said, either in this thread or the Justice League movie thread, there was still plenty of exposition for those who hadn't seen Thor, Captain America or Iron man, things were reiterated--back story was repeated. Like who Thor was, and that Loki was his brother, that Cap had been frozen for 70 years, or that Iron Man is a genius, billionaire, phil-- you get the idea.