How everyone in DC universe forgot about what happen in crisis on infinite earths?

Question

How can they forget the crisis?

Answer 1

Depends on how you look at it, many of them remember a great battle. Certain peices of history could never be explained away, like the death of Dove or Barry Allen (the Silver Age Flash). After the Crisis happened , the universe was merged into one singular timeline. So the battle with the Anti Monitor still happened and those heroes still died. To the heroes who did remember it, it was simply recalled as a battle to defend the new single timeline earth they lived in.
If you ever looked at the Zero Hour Crossover from a few years back, you will notice that most of the older heroes had the Crisis still in their history.

The simplest way to think of it is like the Star Wars Special Editions. Changes happened to the timeline, but the heroes only remember the new and updated version as true history.

Answer 2

Different writers and editors at DC Comics changed things to suit the moment forgetting the larger picture of continuity. A merging of the various universes occured so that there were reboots. However not all characters could be explained by reboots very well. Sometimes the Retcons sucked. Thus we had a number of other crossovers to change things.