James Cameron was determined to make the movie as historically accurate as he could, as we already know. So it makes sense that this extended to the wardrobe department. Getting the costumes done perfectly was extremely important to the director. It ultimately took the costume department an entire year to finish making the clothes for the whole cast.
To add more historical accuracy, some of the fabrics were from original garments from the time period. Obviously, a vintage wardrobe didn’t come cheap, but that didn’t bother the director. He spared no expense for perfection. Cameron also didn’t worry about damaging the costumes when he asked all the actors to go in the icy water while dressed in full costume.