From the wiki: "Unfortunately, as krogan society became more technologically advanced, so did their weaponry. The end result is that they destroyed their homewo

From the wiki: "Unfortunately, as krogan society became more technologically advanced, so did their weaponry. The end result is that they destroyed their homeworld in a nuclear war ... The krogan were reduced to primitive warring clans struggling to survive a nuclear winter of their own creation, a state that continued until they were discovered by the salarians two thousand years later."