You could make this planet an ancient colony of some forgotten race, who disappeared thousands of years ago for some reason (disease, civil or galactic war, ascension to a higher plane of existence...).

Metal is generated from fusion of lighter elements in the core of giant stars ; the one we find in the planet's core and was brought to the crust through billions of years of geological activity. It only takes a few centuries for a civilization like ours to exhaust any mining ressource accessible without tremendous effort.

In short, your colons came to an already exhausted world, and mining the last remains of metal would be way too expensive to be of any utility, except for luxury products.

You could make this planet an ancient colony of some forgotten race, who disappeared thousands of years ago for some reason (disease, civil or galactic war, ascension to a higher plane of existence...).

Metal is generated from fusion of lighter elements in the core of giant stars ; the one we find in the planet's core and was brought to the crust through billions of years of geological activity. It only takes a few centuries for a civilization like ours to exhaust any mining resource accessible without tremendous effort.

In short, your colonists came to an already exhausted world, and mining the last remains of metal would be way too expensive to be of any utility, except for luxury products.