User blog comment:Tiredman2/Mass Effect Plot Holes and Contradictions/@comment-5337704-20120822123143/@comment-4721065-20120824084036

@Temporaryeditor78: Keepers kill themselves after eacdh failure and we DON'T know how long their lifespan is or how fast do they reproduce. It might well be that they reproduce once in 100 years and that 20 Keepers is the minimum required for maintaining the Citadel. In this case, trying harder like you suggested is out of question and it is more then understandable why there was no breakthrough until Chorban.