User blog comment:Legionwrex/What do Reapers do with the technology they harvest?/@comment-3228423-20121218060826/@comment-5379655-20130106031139

It has been referenced to several times in the games that the Citadel races were very comfortable and complacent. They probably just spent a lot of time improving existing technologies over developing new ones. Humanity's entrance to the wider galaxy was like a shot in the arm. On a side note, stagnant technology developed along predictable lines was one of the Reaper's strategies for ensuring easy harvests.