> unaccountably

Maybe because in ASCII-63 the _ codepoint meant left-arrow (which was preserved in at least smalltalk systems much past the 60s)?

My theory is that they might have been trying to protest against the non-mathematical theft of = (equality) to mean assignment, as = is used in most other languages. (Not that that's an excuse for violating the shit out of the Principle of Least Surprise in a trillion different ways.)