[lukan]

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).

[Archelaos]

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-...

[lukan]

Anything in particular in there, that says rather wait for a mountain pass to become unpassable, than leave some meat behind?

(I don't claim this is what happened, I claim a army sometimes has other needs and might have already all the food it can carry)