The usual reason is that you don't have it yet. It's around a level 10 spell IIRC, whereas Teleport is available from the beginning. Once you have it, Feather Fall is flat-out superior since it also costs fewer APs.
I suppose a corner-case situation might be that you need to move two friendly targets at the same time, since Feather Fall would be put on cooldown with the first one and you'd need to use Teleport for the second.
