When you enter into a creative field and want to become successful what you need is a brand value. There are many celebrities who strive to earn that and the same applies to the music directors as well....
Though you see a lot of experiments and permutations happening in the film industry, certain things have been followed as standard procedure. One such aspect has been one movie used to have one music director....
The quickest way to grab attention of the crowds for an event is to come up with a theme song or a title song and if luck permits then it would go viral and create enough hype....