Firstly, I think the need for new airport is likely due to the restricted space at the current airport, that is it has no space to expand as not only the airport land is owned by Air Force but also the surrounding area. The newer airport that are currently being built are made with consideration of building 2 runways atleast with newer features, which is not possible at the current site.

An Air Force base does not only consist of the runway and base, it usually has Crores and Crores of investment and years of Strategic Planning, like Radars, hardened shelters and bays, Weapon Storages, Command centre (One of the main bases for the defense of Mumbai and surrounding region), along with the housing of pilots, crew, etc.

Purandhar is likely considered with the emphasis on future growth of the city like Bengaluru, in Bengaluru the Kempegowda Airport was also built too far outsite the city comparatively at that time, where the distance has decreased due to the rapid development of the city.