Gauranga Mahaprabhu sometimes distributes the task of delivering the fallen souls to His associates because He becomes so absorbed in relishing Radharani's madanakhya mahabhava, like in His pastimes in Gambhira. Mahaprabhu has an internal and deeper cause of His advent which is the primary reason of His appearance. It is described in CC that distributing Harinam and delivering fallen souls is the eternal cause.
However Nitai's mercy is so overwhelming, He cannot stop giving the fallen souls first priority even while relishing His own mood. Even though Nitai is also relishing the confidential mood of His own madhurya swarupa Ananga Manjari, He nevertheless does not give up His mammoth task of running behind the bewildered Kalihata jivas even for a single moment. The driving force behind Nitai's appearance is the excruciating pain He feels seeing the pain of the downtrodden jivas. No wonder Gauranga Mahaprabhu declares that He performs all His tasks through Nityananda! Even the task of distributing ujjwala rasa sarva rasa saar to the souls of this material world!