This is a very good question. The first time I read Julius Caesar, I was shocked that the assassination took place so quickly.
Shakespeare, however, is able to sustain his reader's attention by asking a profound question. Who is the tragic hero? For those who think the tragic hero is Caesar, they will find it odd that he dies so early. If, however, the reader sees that the tragic figure is someone else, like Brutus, then the play begins to make more sense (and becomes very interesting).
Brutus has divided loyalties. He genuinely loves Caesar, but he also loves the Roman Republic. So, even if the other conspirators join the conspiracy on account of envy, Brutus joins the conspiracy on account of love for his country. Moreover, it is so hard for him to do because of his loyalty to Caesar.
Based on this point of view, the reader desires to know what will happen. His or her interest remains to the end to see the character of Brutus.