That's interesting in that when I started playing the ball were set 6" apart. My local room still does that, but I remember sometime in the 90's it seemed the change was made. I'm not arguing with you, its just what I experienced.
My takeaway not surprisingly is that many of the rooms didn't know the rule and 6" sounds good or was stated in their 50year old rules book.
We're likely involved with the same guy.