2G spectrum is less costly and serves basic needs of customers, remember many cannot afford paying high amount for 4G packs.
3G subscribers can easily be migrated to 4G without feeling much pinch and improve overall experience. Airtel has made huge investment in 4G hence it makes sense to do this, 3G is just wastage of money without any major advantage to company or subscribers. Airtel will free the 3G spectrum and put these funds for enhancing other things or provide better plans.
This shift was happening /is happening since last 2 years and all part of their strategy to optimise use of assets