There are a lot of theories about this. We know what some of the factors are, but we can't usually establish a chain of causality to accurately explain why. I'm excluding conquest here because the disappearance of conquered civilisations is usually recorded by their conquerors. The main factors for disappearing civilisations are usually resource related. Either a major resource that the civilisation relied on dried up, or climate change made a large civilisation break down and it's people revert to pastoralist lives (this has been well recorded in neolithic and bronze age middle East ). Sometimes a civilisation could just grow too big to support itself.