We're speaking practically rather than technically here:

Nearly 90% of humans live in the northern hemisphere. People who were developing mapping and navigation techniques did so primarily in the northern hemisphere. So for them, 'north on top' was not arbitrary at all but rather very practical.

If you were holding a map, and trying to figure out which way to orient it so that it matched your surroundings, you'd want to have some sort of reference 'arrow' to line up with something. Historically, the most common physical reference point used in navigation was the north star.

So, face the north star while holding the map oriented correctly and north will be on the top of the map.

My guess would be that navigation in the northern hemisphere relied heavily on Polaris (the north Star), so the 'north arrow' would point in the correct direction if you were holding the map while facing the north star.

It would also coincide with the direction a compass points...