For many reasons, the period of the late 50's through early 60's promoted a clean look.
-- as mentioned, the films you note were shot on 65mm film or Technirama (8 perf 35mm).
-- films were generally much better lit, which meant that there would be less grain and better color. Scenes that would look dark and lifeless today often looked very bright and colorful. [Take a look at the night scenes in The Ten Commandments for example.]
-- the "artistiic" use of grain, desaturated color, etc., was either not done or heavily criticized when it was.
-- epic films and roadshow presentations paid a lot of attention to costumes, sets, and splendid looking photography.
-- process shots were used when necessary but people preferred actually building things and using large crowds of real people. Look at The Fall of the Roman Empire compared to Gladiator (the same story!) for example. Empire actually built enormous sets, Gladiator used CGI that looks very crefty by comparison.
Everyone was aware that they were competing with TV and they had to make films look bigger, cleaner, and brighter to compete.
Beginning in the late 60's, a more "cinema-verite" style was used: grain, shakey cameras, poor color, poor lighting were often considered more artistic. This trend has to some extent continued to this day.