I personally think it's because of the heavy use of white on the older suits. For some reason that helps the colors stand out more, and plus the designs are, like others have said, more tech-based and simplistic, which makes them look better, as opposed to having TOO much details which makes the things look distracting or gaudy (Hurricanger and Dekaranger come to mind, personally), I think that Boukenger got things right by emulating that mid/late 80's style.