I think it's history: in the 1960's, machines were strongly card (or at least line) oriented, so pure right to left intracard yields a very small "parser" compatible with traditional mathematical notation, and the unavoidable intercard transitions become top to bottom on a teletype.
(FWIW in my array noodling, the equivalent of dfns are given in the haskell style of "expr where defs" instead of the strictly temporal "let defs in expr", which does result in a consistent right to left for those who wish to read bottom-up.)
(FWIW in my array noodling, the equivalent of dfns are given in the haskell style of "expr where defs" instead of the strictly temporal "let defs in expr", which does result in a consistent right to left for those who wish to read bottom-up.)