Effectively Wild did a long dive into the rule's history and it's old. Like it started in the 50s, maybe? But it was implemented in such a scattershot way with no umpire buy-in, no penalties, no clock... everyone just ignored it.
Which wasn't really a problem until the 2000s when games started to drag out and kept dragging out. But by then, the rule was ancient, no one observed it, and there was no impetus to start observing it.
Which is why the new, much more clearly written, rule is now in place.