It's not easy creating motivation to save an empty, lifeless galaxy. I'm talking about losing the suspension of disbelief, taking out the implied habitations, and looking at the numbers. How many people did you see on earth? Maybe a few dozen marines, bustling through the military complex, and a small gathering of the ruling officers and their lackey's when the Reapers attacked. Then one or dead humans here, a small child there, and then a cityscape with millions of implied lives.