The Mountain was saved by Qyburn, who was kicked out of the maesters club for unethical experiments on living humans. He wasnt around when Joffrey was poisoned, and whats more Joffrey did not live long enough to be saved even if he had been at the party; the poison used on The Mountain was designed to keep him alive (in horrific agony) for some time before he died.
Also one has to temper ones definition of 'revived' when discussing Gregor, since clearly Qyburn basically Jeffrey Dahmer'd him to some extent and now hes some sort of hugely strong automaton. Who knows if he even has a head under that helm anymore.