Why do you think Kisa Gotami understood this only the second time? In what way did the Buddha change her understanding?
When Kisa Gotami went to her neighbours, she was only thinking about her grief. She was asking for a medicine that would cure her son. Being refused by everybody, she came to Buddha. He asked her to get a handful of mustard seeds from a house where no one had died. He wanted her to realize that no such house exists where no beloved had died, and that death is inevitable. She could not gather the mustard seeds and felt dejected. Then, she sat down at the wayside watching the lights of the city. Seeing the lights flickering up and extinguishing, she realized that the fate of men is such that they live and die. She had now understood that death is common to all.