Because they might be in a hurry to reach that mountain pass before the snow comes. Or to meet other goals set by the general and if already stocked up on supplies, it would be quite a burden to also carry an extra elephant (which was likely also carrying things before it died).

Quite unlikely. For an army on the move, especially one that had to cover such a long distance, a considerable portion of the troops was assigned to procure provisions anyway.

Bret C. Devereaux has written a series of three blog posts on the topic of pre-modern army logistics that explains that in detail. See: https://acoup.blog/2022/07/15/collections-logistics-how-did-...