REasons for commercial success
One of the reasons for widespread commercial success for many art forms and media is the ability to appeal to a very wide group of people. For instance, commercial chart albums are very often conceptualized in a way to include not only a wide variety of styles but also include songs that are musically more challenging for the better educated and musically “sophisticated” people and songs that work well with the general audience due to easy understandable musical ideas and catchyness. The same strategy can be used in composition. The popularity of John Williams has also to do with the fact that he writes incredibly multi layered music that includes attractive elements for many groups of people. The striking melodic simplicity of some of his themes speak very well with a general audience, being able to hum or sing the tune in an instant and being musically satisfied with the catchyness of the main idea while musically educated people and music lovers find attractiveness in the detailed orchestration, adventurous harmonic paths and extraordinary craftmanship in the very same pieces. Many young composers (especially the conservatory trained) often try to write complex music on every level which of course speaks often well with their (former) professors and music elitists while the general audience is simply overwhelmed by the complexity. If your approach however is to reach a widespread audience (which probably most people want), simplicity is nothing to avoid but strive for. Basing a composition on a very simple idea but on top of that finding ways to make it sophisticated is actually not an easy task but will result in something that has a higher chance of appealing to many people. Of course the definition of simple and sophisticated in itself is something to discuss about and seen very differently by different people but the strategy behind that has proven several times to work very well.