8 bits is all that's needed to address the fixed 1 MB of memory in bank A, arranged as it is using 256k x 4 chips.

2^(8 row addresses + 8 column addresses + 4 bits) * 8 chips == 8 megabits == 1 megabyte.

Why bother routing more lines that are just going to force you to allocate more gates for buffers and multiplexers, add to the capacitive load, and take up valuable PCB space, while serving no useful purpose whatsoever?