In Medieval Europe (say from about 400-500AD till the 1400s), the orientation of maps with East at Top was a convention that took hold, with only a few exceptions either side of this time frame. But this was so only during this period. It was by no means a convention before, indeed there was no real convention before, and certainly not after.
The three main, somewhat overlapping, reasons commonly cited for this are: a) Jerusalem was in the East and, with the vast majority of maps being created by people of a Judeo-Christian religious persuasion, that's where the importance lay. These were also largely Crusading times, after all. b) The sun rose in the east and c) The Garden of Eden was supposed to be somewhere in Asia and was of course of prime importance for the Christian mindset. You can actually see Adam and Eve with the Serpent depicted at top (ie. in Asia) on many of the Christian maps shown in the last group I coughed up.