Because backslash is a modern invention with no prior meaning in text. It was invented to allow writing the mathematical "and" and "or" symbols as /\ and \/.
Hm. According to Wiki, "As of November 2022, efforts to identify either the origin of this character or its purpose before the 1960s have not been successful."
While your rationale was used to argue for its inclusion in ASCII, as an origin story however it is very unlikely, as (according to wiki again): "The earliest known reference found to date is a 1937 maintenance manual from the Teletype Corporation with a photograph showing the keyboard of its Kleinschmidt keyboard perforator WPE-3 using the Wheatstone system."
The Kleinschmidt keyboard perforator was used for sending telegraphs, and is not well equipped with mathematical symbols, or indeed any symbols at all besides forward slash, backslash, question mark, and equals sign. Not even period!
"Why character X for Y?" has usually a universal answer: character X wasn't used commonly for the given scope at the time, was available widely, and felt sensible enough for Y.
