in scene 1, why does cassius argue against allowing antony to speak at caesers funeral?

Cassius argues against Marc Antony speaking at the funeral. He realizes that Antony can so easily turn the people against the conspirators. Brutus disagrees. He explains to Cassius that he will speak first, and then give permission for Antony to speak. It will appear that Antony is being submissive to the conspirators. This will strengthen the conspirators actions.

Hindsight is twenty, twenty, to borrow a cliche. Cassius was so right. Antony's speech stirs up the crowd to an angry frenzy, an absolute hysteria against the conspirators.

Brutus and the conspirators have to flee for their lives.

Antony's speech has served its purpose. The people are rallied against Brutus and the conspirators. Cassius was wiser than Brutus, but Brutus was in charge.