Probably a limitation of the color film stock at the time. Technicolor used a beam splitter and three strips of black-and-white film. It made super-vivid colors but was expensive. Single-strip color film rose to the top in the 50s and 60s with worse colors, but much cheaper and easier to shoot.
The Adventures of Robin Hood, the Errol Flynn movie from 1938, is an excellent example of three-strip Technicolor, and on blu-ray some scenes look like they were shot last week, not 80 years ago.
