If they had picked the correct grade of stainless steel, it wouldn't rust just by being outside. Cheaper grades rust, higher grades don't.

Fun fact: the soviets simply used thicker steel. That's partly why there's so many of those shitboxes still riding around the ex-soviet block. Based on the Fiat 124, which often rusted and died in the Italian sunshine, the Soviet variant often rusted but survived in Siberia.