They got tired of waiting to play each other.
The ACC and SEC each have 14 teams, 8 conference games, divisions, and a permanent cross-division game.
For example, each year Alabama plays Arkansas, Auburn, LSU, MSU, Ole Miss, and TA&M from the West and Tennessee from the East. The last conference game rotates every 6 years between Florida, Georgia, Missouri, South Carolina, Tennessee, and Vanderbilt. So it takes 12 years to play in every stadium in the conference.