system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] Why didn't the Reapers shut down the relay network when invading our galaxy?
gpt
They would have, but Prothean meddling prevented the Reapers from executing their decapitation strike. Their backup plans failed as well because the Alliance navy destroyed Sovereign and Shephard destroyed the Alpha Relay (which was a special relay capable of sending a fleet directly to the Citadel, bypassing the relay network).

Without the Citadel, the Reapers had to manually reach the borders of the galaxy and fight their way in through the relay network. This necessitated the occupation of a large number of star systems, as to simply beeline for the Citadel would have left the Reapers with an outstretched force surrounded on all sides by opponents.

That scenario again changed when the Illusive Man revealed the existence of the Crucible to the Reapers. The superweapon would only function if connected to the Citadel, so the Reapers had now a large incentive to abandon their slow, steady caution and assault the station to then move it in Earth's orbit. A large reason why their assault was so easy was also that they had already destroyed large portions of the Galaxy's military and had pinned down much of the rest in thinly spread fighting.