I came to the conclusion that Sarantha is the main reason why humanity has always failed to prevent the Cycle, as a being that essentially controls the fate of the mortals, in fact as she act to preserve the balance of the universe she created the Cycle in order to prevent a civilization to become too powerful and thus altering the balance.
The previous Shadow Gods have always failed to defeat Sarantha so every Cycle was inevitable.
Probably this was the first time that the Cycle could be prevented and all the downfall of the human civilization to wars, people gone berserk and undead lost ones.
If you managed to read the letter of the Aged Man to his friend in Nehrim where he initially expresses optimism for the latest events (the death of the Light Borns and Sarantha?) but then he says that nothing has changed in reality I suspect that he initially thought that with humanity free from the influence of Sarantha this time could prevent the Cycle and avoid to fall into the same trap but once is evident that the Cycle is turning its gear as it has already happened with the previous civilizations he becomes totally disillusioned.
Even now that humanity is free cannot stop in doing the same mistakes.