An entirely speculative theory of mine is that it's correlated with our significantly weaker senses of smell- given that smell is the sense most strongly correlated with memory in hominid populations. People in hunter gatherer tribes tend to have much better senses of smell and better memories. Chimps have much better senses of smell yet. Of course, there's also the whole "writing degrading memory" theory to reckon with as well.
And thanks, glad you enjoyed it!