Some bands have piano, and Billy Joel has harmonica, but for the most part, rock = guitars. Why is this so? Why didn't Metallica have a trumpet player, for example? What was it that led to the guitar becoming dominant over all other instruments?
You will be pleased to know that this subject is dealt with at length in an episode of the AskHistorians podcast where I was a guest - hopefully there's more than enough information there to answer your question!