I would argue that, story-wise, the Reapers seem focused specifically with humans since the Collectors were using human DNA to construct the Reaper at the end of Mass Effect 2. That could explain an increased emphasis on Earth in this game.

Also, the Geth already attacked the Citadel in Mass Effect 1. While the Citadel would seem like a more likely setting for the climax of this game because of its role as the political and social center of the Mass Effect universe, and the Reapers are protrayed as a galactic thread, it might come off like a repeat of the first game.