I would guess that since wiki is less of an impulsive stream-of-consciousness type of a medium and more of a "sit down and write things in an organized manner after thinking things through" type, people are much less likely to put something that could create a problem for legal there (as opposed to Slack).

Not sure how factually true this is, but that rationale makes perfect sense in my head for why legal would make that rule for Slack/messaging while being totally cool with persistent wikis.