Because every animal who came to visit the stag ate the grass.
It began on the very first visit. The animals wished the stag a speedy recovery, “but before they went back into the forest, the animals ate the fresh green grass growing on the hillock”.
Then they told others how good it was. The elephant said, “The grass is so tender and nice.” The giraffe said, “I have never eaten such juicy green leaves.” The story says, “Word soon spread about the tender green grass” — and after that, “anyone who came to visit the stag would also eat the grass”.
Why it vanished so fast: The hillock was small, and grass takes many days to grow back. A few animals ate a little each, but so many came that the little bits added up. In the end there was not much grass left for the old stag himself.