User blog comment:James12708/How could no one know about the Reapers/@comment-4950455-20120816084827

Keep in mind that everyone assumed the Protheans were the first true galactic civilization, the ones who built the relays, etc. If the people of Citadel civilization knew there had been lots of previous civilizations similar to theirs, that would have been a bit more worrying. But one vanished precursor race and lots of tiny little civilizations that all died out? Not as suspicious. Every civilization ends eventually, and without relay travel, there's little mystery about why no one ever heard from them.

Incidentally, while we're on this topic I want to take a moment to express my admiration for Bioware planting in all those little clues. It was a great moment when I was able to look back and realize that's why such an awful lot of planets have Codex entries about showing "signs of orbital bombardment".