This is a good question. Antony is addressing a hostile audience. Brutus has just finished speaking to them and has won them over to his cause. Notice the dialogue among the plebeians after Brutus' speech, including the following:
Give him a statue with his ancestors.
Let him be Caesar.
Caesar's better parts
Shall be crown'd in Brutus.
Then when Antony starts to speak after being introduced by Brutus, the plebeians make it clear that Antony had better not say anything against Brutus. Antony has no intention of doing so--at least at the beginning of his speech. Later when he has the mob on his side, he changes his tune. He refers to Brutus and all the other conspirators as "traitors." Here he uses the words "honorable men" ironically.
I fear I wrong the honorable men
Whose daggers have stabb'd Caesar; I do fear it.
They were traitors. “Honorable men!”
Throughout his speech he is in tune with the spirit of his listeners. He has his work cut out for him, because he has to turn the plebeians completely around. They are sold on Brutus. Brutus could become king at that point if he wanted to.
He says, for Brutus' sake,
He finds himself beholding to us all.
Twere best he speak no harm of Brutus here.
Antony is cunning. He realizes that he had better proceed with caution. By repeatedly saying that Brutus is an honorable man, he appeases the mob and gets them to listen to him. Besides that, Antony truly believes that Brutus is an honorable, though misguided, man. When Antony and Octavius defeat Brutus at the Battle of Philippi, Antony pays his enemy this tribute.
This was the noblest Roman of them all.
All the conspirators, save only he,
Did that they did in envy of great Caesar;
He only, in a general honest thought
And common good to all, made one of them.
His life was gentle, and the elements
So mix'd in him that Nature might stand up
And say to all the world, “This was a man!”
Antony doesn't feel there is any need to revile Brutus or any of the other members of the conspiracy. He takes an entirely different approach. Brutus in his speech appealed to their reason. He wanted to quiet them down, quell any possible civil disturbance. Antony, for his part, wants to do just the opposite. He wants to appeal to their emotions and to stir up a mutiny. In his soliloquy, in the previous scene, he makes it clear that he intends to cause trouble if he can. Unlike Brutus, Antony does not wish to pacify the Roman masses but to stir them up. He uses his eloquence to do so. He also has two things to show the mob which are certain to have a powerful effect. Shakespeare learned them both from Plutarch. One is the corpse of Caesar. Brutus wants the citizens to think it was a sort of necessary sacrifice, but when they see the mutilated corpse they realize that it was, as Antony describes it, butchery. The way the conspirators hacked at Caesar speaks for itself: it shows that they were motivated by anger, hatred, envy, jealousy, fear, frenzy, and other unworthy emotions. This spectacle appeals to the plebeians' emotions. The other exhibit appeals to their greed. Antony has Caesar's will, which leaves generous bequests to all the Roman citizens. He uses that as his principal means of stirring up a riot. After they all depart to burn the houses of the conspirators and spread the mutiny all over Rome, Antony says:
Now let it work. Mischief, thou art afoot,
Take thou what course thou wilt.