In this scene, it was Casca who said "Speak hands for me!" and was assumed to be the first to stab Caesar. There were a lot of conspirators and the order in which they stabbed Caesar wasn't really specified. Caesar saying "Et tu, Brutè?" would make people assume that Brutus was the last to stab Caesar. This was also the most impactful as Brutus was Caesar's closest friend and the betrayal was striked Caesar greatly.