The crota we see is a projection of his true self. Oracles would erase just the individual projection. Another would come along and it would keep going until Atheon is killed. Not to mention Crota could not be hurt by Atheon. There is also the matter of the hive entities being tied to different dimensions like the vex are. As such oracles would likely not affect him because he would be indistinguishable from other vex to them. Just like the deathsinger has woven the hive into her song rendering them immune to its effects.