Why did Augustus delay Tiberius’s return from Rhodes?

Augustus delayed Tiberius’s return from Rhodes primarily because he favored his grandsons, Gaius Caesar and Lucius Caesar, as his successors.12

During Tiberius's self-imposed exile, these young men were introduced to public life with great fanfare and were clearly positioned as the primary candidates to inherit the principate. Augustus only relented and permitted Tiberius to return to Rome in 2 ce, after the death of Lucius in Marseille. Following the subsequent death of Gaius in 4 ce, Tiberius became the most viable successor, leading Augustus to formally adopt him.12