Humanity has growing approximately exponentially in numbers. Due to the inherent nature of the exponential process it means that a fixed percentage of all people that have ever lived are still alive. This is turn means that the proportional of historical writing to contemporary writing is approximately fixed. With exponential growth, remembering everything or at least everything significant is sort-of viable. But if humanity is about to stabilize in numbers, which would be desirable, at least until we can expand our society beyond Earth, then it means that the proportion of history to contemporary will steadily increase, and a sort of willful amnesia will become more important. With fixed resources we will have a fixed ability to remember and most pick and choose.

And that's why we have to colonize space. To post-pone amnesia.