i think it's the moment where bioware realized that the ending was terrible. if they would like to do the same like with previous games and import saves then the story has to be completely different for every each ME3 ending. so lets face it - out of the question.

it's easier to make a prequel where you don't have to account for the variety of choices you've made during ME1-ME3, especially the last one since it had the biggest impact for the future story. there was always a canon approach to the game (how the writers saw the story roll) but they also gave you a sense of continuity by imports. now? it's impossible so they don't want to give you an option.