Writing top to bottom, and even left to right has/had advantages for mostly right-handed writers to avoid moving your hand over and smudging previously written text.
Writing top-to-bottom has advantages for all writers whose eyes are above their hands. The bit of the writing surface that's blocked by your hand hasn't been written on yet.
Extending that, heads have a much easier time moving left and right than up and down, since the motion uses the pivot joints. So, that means rastering left to right, then top to bottom, is the best match to the average reading and writing human (since right handedness is the dominant genetic trait [1]).
Top to bottom advantages everyone. Left to right advantages the right-handed. Right handed being the majority, top to bottom and left to right wins in almost every writing system.
