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.
Most of our geographic and astronomic terms were developed in pre-history in the northern hemisphere. Even early astronomers noticed that Polaris, the north star, was 'up' in the night sky, so north as 'up' simply became the convention.