Original: The plan fails due to the interference of the Doctor, his companions and Caan himself, who has been manipulating events to destroy the Daleks after realising the severity of the atrocities they have committed.
Simplified: Davros and the Daleks planned to destroy all creation with a 'reality bomb', which failed due to the interference of the Doctor and his companions, and due to Caan himself who had been manipulating the events unknown to either side.