Amy, an excellent point. He is the catalyst, the "star in the firmament" and fixed and true and reliable. The other characters are more reactive and take unreliable actions. Question for Maria, if you're still following this: why do you think acts 4 and 5 focus on acts of war, with Brutus leading his troops and disagreeing with Cassius, while Antony and Octavius struggle in their ranks, but eventually emerge victorious? What do these events say about the importance of both Caesar and other characters such as Brutus and Antony? (I wouldn't leave out Cassius, either.) Shakespeare was interested in the complex characterizations, I believe, which is part of what gets at your question.
Everything happens in the play because of Julius Caesar. He may not have an active role in the play after his death (disregarding the appearance of the ghost, of course), but he is responsible for the motives and actions of all the other characters. Cassius, Brutus, Octavius, Antony, Calpurnia, everyone...they act and react because of Caesar and the actions he put forth both before his death and after --the result of many men reacting to his death.
A great question to explore! First, some historical context: growing up in Elizabethan England, Shakespeare most likely attended grammar school, where boys mastered the history of Ancient Rome. The star was Julius Caesar. Theatergoers coming to see Shakespeare's tragedy would already know Caesar was a great general and leader who died tragically at the hands of conspirators. So Shakespeare doesn't need to take much time to prove that Caesar is great; he needs only a few moments such as Marc Antony's grieving over Caesar's corpse ("...the noblest man/That ever lived in the tide of times" III, i, 256-257). It's generally believed that Elizabethans would need no more convincing.
So if Caesar is a great man, the hero who is butchered by Act III, then can there be another hero? That's what makes Shakespeare so wonderful: his ability to allow two protagonists to share the stage. How do we know Brutus is great, too? What makes him outstanding? Yes, he kills Caesar, but why? What does Antony say about him in the final lines of the play? Finally, what is Brutus's struggle throughout the play? Answering those questions get to the psychological complexity of Brutus that captured Shakespeare's imagination. Brutus has a conflict and that makes for a powerful tragic hero.
It's also worth noting that Caesar, though somewhat of an icon in the play, has complexities and frailties, too. They aren't as compelling as those of Brutus who gets time in act V.