Metalbenders don't actually bend metal. What they're doing is manipulating the microscopic amounts of earth inside the metal. Remember that bending is equally spiritual and physical, so there has to be a magical connection to the element for a bender to work with. And imprecise or shoddy methods of separating metal from its ore can leave a great deal of spiritual earth inside its structure to be bent.

The method that Future Industries (and later Kuvira's factories) used to turn platinum into sheets of metal removed all of its spiritual connection to the element of earth. There is no impurity for the bender to latch onto and move, so the metal is immune to their abilities.