i think, this is just my guess, when there were very few trains from varanasi/MGS towards Delhi/Ambala, at that time MGS division (then with ER) used to attach an AC coach at MGS. so that people of that region can reach Delhi/ambala.
Now as MGS is in ECR division, and there are many trains between Varanasi/MGS towards Delhi/Ambala and also due to coupling and decoupling a platform is occupied for more time. so ECR decided to stop this practice.