I think, it may be worth keeping in mind how important it was to have an intuitively accessible representation in the era of console lights and toggle switches. Ones' complement and octal grouping (it's easy to interpret a triplet of bits) did this. – And subtraction was just addition with one of the operands inverted. On the other hand, you had to compensate for this by adding extra circuitry to handle transitions across zero (i.e., a change of the sign-bit occurring in additions or subtractions), and checking for -0 in code as well. As operator consoles became less important, this became less worth the effort and we transitioned. (Mind how the connection between octal digit positions and bits is lost in two's complement. This would have been a major issue.)