Two reasons I can think of, both Watsonian and Doylist.
Watsonian: it's the direction of local prevailing winds. Possibly not strictly north/south, but there's no reason the game map has to be oriented with north up.
Doylist: a minor disadvantage to balance it's minor advantage of still only needing 2 polymers to upgrade (and being slightly more efficient with drone time, since maintenance doesn't use half-unit chunks)