User blog comment:Tiredman2/Mass Effect Plot Holes and Contradictions/@comment-1978336-20120821041252/@comment-4950455-20120821062136

I'm not sure that fully explains it. The reason Citadel space is much smaller then it is because of how the relay network functions - instead of one large, continuous territory, you get little "puddles" around each mass relay. The problem here is that Bekenstein is in one of those puddles; while I'm not sure about exact distances on the galaxy map, it's definitely very close to the Widow system and the Citadel. It does seem odd, given its location, that it's gone completely unnoticed for over 2,000 years.

(Equally strangely, I think it's mentioned pretty early in ME3 that the Reapers have attacked and destroyed the colony, despite it being pretty far away from the region they've penetrated as of yet).