This question popped up in my head while watching LOTR, in wich a tree says: going south always feels like going down.
There is no good reason. It's simply a convention that someone started a long time ago and that stuck. Like the question whether an hour is divided into 60 minutes and a minute into 60 seconds.
If you go to Australia, you will find plenty of maps for tourists that show Australia at the top and Europe at the bottom.