@sophia
I would strongly guess that this is (mostly?) a matter of frequency response.
(some discussion here https://en.wikipedia.org/wiki/Panchromatic_film)
A b/w silver-halide film will have a some kinda frequency sensitivity that I haven't been able to find any charts of, spanning the entire visible range and a bit beyond on either end.
@DMN yeah I've heard that before, among other reasons. The end result is the same regardless; b/w on digital is ass.
@sophia
Whereas every digital camera I've ever seen had three sensors, each with their own much narrower sensitivity curves (although they do typically extend into ultraviolet), and when you set it to "B/W" it does some kinda averaging function; so the final response would be three sharp peaks instead of whatever "nicer" curve modern silver halide gives.