Harold Camping already failed to predict Judgement Day (technically he is predicting Armageddon, Judgement Day comes after the Millenium) back in 1994, so I am not sure why anyone still follows him.
Jehovah's Witnesses' have also predicted the End several times incorrectly (although, as with Harold Camping, proponents feel they were steps towards the End) and they still have many followers.
Its hard to admit you are wrong and then turn back on some basic beliefs. I don't envy the position his followers are in.