Durga is very much an aspect of the Divine Mother just as the great Gods Shiva, Vishnu, Brahma and the rest. In fact seen from the Mother’s side whole creation is part of the Divine Mother. But coming to the gods and goddesses each of them have a certain function to perform which they spontaneously do as typal (non-evolutionary) beings. This continues until the cycle of Time changes. When this happens the gods and goddesses too must recalibrate, modify their function keeping in consideration the new age and Time that is coming. Great though they are the gods and the goddesses have a certain range and type of function. In their sphere of action they are absolute, yet they are not beyond Time, they are still subject to the Mahamaya and they cannot know the entire Divine Play and the Divine Will unfolding in creation. Even the greatest of goddesses Mahakali changes time but at the behest of the Will of the Supreme Mother, the Ishwara and His Shakti. Durga is of course the combination of Maheswari and Mahakali who protects, guides and drives the Asuras back to their den.
Now each of these divine beings are very powerful and feel absolute in themselves. They are impelled by the Divine Law binding on their nature and do not feel the need to surrender. After a cycle is over they draw back into the Supreme Mother and are cast again modified for the needs of the New age. This is what happened when the Mother asked Durga to surrender and other great gods if they would collaborate in the New Creation. This time there would be no pralaya hence the Divine Action would be of a different nature, conversion or transformation of the Asuras, rather than their destruction or driving them back into their den. This time it is the highest Divine Love at play working for the transformation hence the role of the four great goddesses (described in Chandi) is changed into preparing man for the transformation. Durga too has to undertake a new mode of working (The Mother describes that as well), of removing certain barriers and fences rather than right away driving away the Asura. It is a new Lila so to say.
We see this kind of shift happening to the gods during Ramavatar and more clearly during Sri Krishna Lila whence Indra and Brahma both are humbled when they interfere with the Lila. Even Hanuman ji is requested to just be there rather than show his tejomaya roopa. So too during the Trojan war. This time even a greater shift is needed which the Mother is refering to in the story of Durga’s surrender to the Supreme.
Affectionately,
Alok Da
