Wriddhiman Saha was out of the team for a long time due to injury and came back into the side with the two Test series in the West Indies in August. Of late Rishabh Pant has been criticised for reckless strokemaking and that may have been a factor in the team management's decision to start with Saha.

from Times of India https://timesofindia.indiatimes.com/sports/cricket/south-africa-in-india/1st-test-rishabh-pant-loses-his-place-in-indian-team-wriddhiman-saha-to-keep-against-south-africa/articleshow/71389121.cms