Furthermore, Mark Antony wasn't the best politician. Caesar knew that, however, Calpurnia (Caesar's widow) did provide Antony with papers from Caesar that both made Antony his interim heir as well as leader of the Caesarian faction. When they opened Caesar's will a few days later Octavius was named as his official successor, but Antony did his best to ignore that.
Why? It's speculation, but Gaius Octavius was young and very bright. Caesar's sister Julia practically raised him. Octavius also traveled across a hostile Hispania to join Caesar's war-time camp, which Caesar never forgot. Lastly and somewhat controversial - Antony spread rumors that Octavius and Caesar were lovers.