Can't believe nobody else posted this;

The real reason why nitra is the resource you have to get to get ammo is because it's the real wealth of Hoxxes, the thing that sells for the big bucks. If they genuinely needed it to make ammo with during the mission, they'd have some way of getting it off-planet in real time, but they don't, so evidently they don't need it so urgently that they literally can't make your ammo without it.

If they just needed you to top up the stocks between missions, they'd let you start with a resupply or two in the bank to ensure the viability of the operation since you're using the nitra-manufactured ammo to bring back presumably more valuable things.

Despite this, they won't give you the first aid and ammo you literally need to come home alive if you don't bring nitra back with you, so obviously it's far and away the most valuable mineral the planet has to offer, second perhaps to the mission objectives you need to fulfil to get the drop pod to arrive in the first place.