Previous Acharyas have already written many commentaries on Bhagavad Gita. Then why did Srila Prabhupada write another commentary?
The Bhaktivedanta purports as Srila Prabhupada‘s commentary popularly called are based on the commentaries of the previous acaryas.
Srila Prabhupada would work from a Bengali translation of the Bhagavatam, with commentaries by twelve acaryas, such as Vishvanatha Cakravarti, Jiva Gosvami, Sanatana Gosvami, Sridhara Svami, Bir Raghava Gosvami, Madhvacarya, Bhaktisiddhanta Sarasvati, Bhaktivinoda Thakura. The Bhagavad-gita is dedicated to and follows the commentary of Srila Baladeva Vidyabhushana; the Caitanya-caritamrita purports are summarized from those written by Bhaktisiddhanta Sarasvati and Bhaktivinoda Thakura. Even if we consider that Prabhupada’s purports are partly translations of previous commentaries, this does not diminish our gratitude for his preaching and giving us access to the thoughts and devotion of the disciplic succession. Once, the devotees were commenting on how quickly Prabhupada was writing. Prabhupada said “O, I can finish very quickly, but I have to present it for your understanding. It requires deep thought, very carefully to present”. (Srila Prabhupada-lilamrita)
And neither were his purports just static translations of other commentaries, but he struggled to apply the words of the previous acaryas to the present-day mentality of the westerners. We can imagine how difficult this must have been trying to present the principles of Vedic government, or the position of women in Vedic culture, or even the necessity to avoid sinful life to persons who were addicted to sense gratification and who had no idea what was wrong with it. He saturated his purports with Krishna, and built the groundwork to supply his readers with the cultural language to enable them to enter into the pages of the Bhagavatam.