Historically most Underground rolling stock couplings were "handed" - a left handed coupling ("A-end") could only couple to a right handed one ("D-end"), and each unit had one of each. (Ambidextrous couplings need some connections to be duplicated or stacked one above the other, both difficult to arrange in the very limited space available on the end of a Tube car).
A common orientation made it easier to arrange stock transfers between lines. (And yes, the loops at Hainault, Kennington and Heathrow do complicate matters).