Why is Marcus Brutus an insignificant character in the play Julius Caesar?

Brutus is roped into the conspiracy, and he does not end up leader of Rome.

Marcus Brutus is known as one of the men who assassinated Julius Caesar.  It is thought that he was the last one to stab Caesar.  Shakespeare’s play draws particular attention to this.  However, Brutus is really nothing more than a figurehead throughout the play and never has any real power.

Notice that Cassius started the conspiracy, not Brutus.  We are not even sure that Cassius started it, we only know that he is the one who convinced Brutus to join.  Brutus was by all accounts a very wishy-washy man.  He has no real leadership skills.

Brutus does take charge of the conspiracy, but his involvement in it was not because of his ability.  They needed his name.  Brutus was from a very old aristocratic Roman family.  He was wealthy and important, but it was not because of anything he did.

However, every decision that Brutus makes is a disaster.  He determines who else can join the conspiracy (no one, or at least not Cicero or Portia), who else besides Caesar will die (no one), and who should talk at Caesar’s funeral (himself and Mark Antony).  He also determines where they will fight later when they have their own armies. 

Every decision he makes is the wrong one.  Brutus came into the plan for the assassination when most everything had already been decided.  The plan was there.  Brutus had to determine whether or not Antony should be killed too, and he decided that he was not dangerous!  He doesn’t want the assassins to be seen as butchers.

This shall make
Our purpose necessary and not envious:
Which so appearing to the common eyes,
We shall be call'd purgers, not murderers.
And for Mark Antony, think not of him;
For he can do no more than Caesar's arm
When Caesar's head is off. (Act 2, Scene 1)

Brutus wants to make this out as some worthy cause, rather than what it is: a political coup.  He doesn’t realize that Antony is actually the biggest danger to him.  Antony was Caesar’s lieutenant, and when Caesar dies Antony cleverly manipulates things so that the loyalty of the people and Caesar’s soldiers is transferred to him.

When Brutus speaks to the people, he is hardly convincing.  His speech is vague threats and a lot of posturing.  It is nothing that is going to get the people to throw their support behind him.  

As Caesar loved me, I weep for him;
as he was fortunate, I rejoice at it; as he was
valiant, I honour him: but, as he was ambitious, I
slew him. There is tears for his love; joy for his
fortune; honour for his valour; and death for his
ambition. (Act 3, Scene 2)

Even his speech is wishy-washy and odd!  Antony is easily able to counteract it with his strong emotional appeals and theatrics.

Even Caesar seems to realize that Brutus is a wimp.  When the assassins all stab Caesar, Brutus seems to freeze.  He is actually the last one.  Caesar’s reaction is shock.  He asks Brutus, “AND YOU?”  In other words:  You would get involved with these men?  He knows that Brutus is not the brains of the operation.

When Antony runs them off, Brutus continues to make bad decisions.  He has no significant military experience and is certainly not qualified to lead an army.  He overrules Cassius, who does know what he is doing, and sends them all to their deaths.  Brutus eventually kills himself in shame, reluctant to be captured by Antony.