You want something that is easy to use for a Z80 (and not to hard to do with the custom logic chip, the "ULA"). Making each line (row) 256 bytes apart makes it

You want something that is easy to use for a Z80 (and not to hard to do with the custom logic chip, the "ULA"). Making each line (row) 256 bytes apart makes it fairly easy to draw characters: copy a byte over from the ROM, increase the ROM address by 1, increase the high byte of the screen address by 1, continue.