Because if they remembered and revealed it to others, it would have jeopardised Sri Krishna’s work, which among other things was to set an example for humanity to follow. Sri Aurobindo reveals that ‘The Divine does not work arbitrarily or as a thaumaturge; He acts upon the world along the lines that have been fixed by the nature and purpose of the world we live in—by an increasing action of the thing that has to manifest, not by a sudden change or disregard of all the conditions of the work to be done. ‘
It is to maintain the limits of the work, to keep it within the needed boundaries that Devaki and Yasoda Ma were made to forget a vision that would have been too overwhelming for them. But when it was needed, Sri Krishna did manifest his Viswa Roop, first as a caution to warn the kauravas and later to remove all doubts from his chief instrument Arjuna, and ensured it was not forgotten.
Affectionately,
Alok Da
