The B1G, B12 & PAC technically already have the rule because they play 9 conference games. The ACC & SEC decided to stay at 8 conference games and so they needed to "in the media" and "as a requirement" establish something that validates their schedules will be as strong of a schedule.

ACC didn't go 9 games because 5 of their teams will already be adding ND each year. And many of them already have rivalries with other P5 teams (many of which are in the SEC) and didn't want to be stuck with 10 or 11 P5 quality games....