User blog comment:Suparockr/List of Plotholes throughout the series/@comment-70.143.64.2-20120723023327/@comment-1221773-20120723024728

You resolved the issue yourself. If the Catalyst did not activate the Citadel relay itself when the keepers failed, then either it cannot control the Citadel or does not want to. The way I see it, the Reapers are an experiment and the Catalyst is an observer; it cannot and/or will not interfere because if the Reapers can't handle things by themselves, they are not a viable solution to synthetic-organic conflict and the Catalyst would have to think of something else.