Bottom+right is purely a performance optimization, when processing top-to-bottom, left-to-right. You only ever push the error terms to pixels you haven't proces

Bottom+right is purely a performance optimization, when processing top-to-bottom, left-to-right. You only ever push the error terms to pixels you haven't processed yet, so the whole algorithm remains a single linear pass. That was the standard approach for handling images at the time. Everything was stored as lines of raster data, rather than the blocks modern compressed formats prefer, and there wasn't parallel processing available to speed things up in ways that make linear data dependency awkward.