In the play Julius Caesar, why was Brutus' downfall not his own doing?

Brutus was responsible for his own downfall because he was naïve and did not listen to advice.

The main reason that Brutus was responsible for his own downfall was that he did not listen to Cassius. He never took anyone else's advice on issues such as how best to assassinate Caesar or how to run the military campaign against Antony and Octavius. Instead, he wanted to remain optimistic. Since he did not, they failed. He was naïve, and he paid the price.