User blog comment:Garhdo/Why the Reapers attack Earth/@comment-5046756-20120520201259

Vigil also states that when reapers take control of the citadel, they also take control of the mass relays, isolating individual star systems from one another. This would have made communication and military coordination almost impossible and the reapers, being synthetics, have the time to slowly harvest the galxy at their leisure. Even if the galaxy was then united to stop the reapers they would either be unable to reach the citadel (or each other), or decimated by the reaper fleet.