"why can we remember the past, but not the future?"
-stephen fucking hawking
Because it hasn't happened yet.
I have no idea why this shouldn't be a perfectly good answer.
Because the guy who wrote the question wrote a much longer and smarter-sounding answer.
I read A Brief History of Time
, but it has been a long time. Yes, Hawking does a good bit explaining what he meant by the question, but the simple answer still seems good if all you want is a brief answer. Of course, if you want to know why cause-and-effect happen in that order, and why we experience them the way we do, that seems to border on metaphsyics, and I'm not sure if science can truly answer them.