There was a belief, before the advent of ubiquitous password managers, that allowing passwords to be "too long" would result in people forgetting their password more often, entering it wrong, or some combination which would increase reset requests and ultimately cause people to use worse passwords. Basically "you can't remember a 54 character random password, and you're gonna get pissed and switch to a six character predictable word".

This is now obviously a terrible line of reasoning, but it was only middling bad at the time.