Germanicus was a Julian, of a good age, a tested and highly successful general, and could surely be taught and excel in administration considering his competence in all other areas. Meanwhile, Tiberius was a Claudius, was at the time exiled and completely astray from the political scene, and his relationship with Augustus was sour. Even though he may have had more administrative experience and was possibly just as able, if not more able, a general as Germanicus, surely Drusus' son was the better option?
The History of Rome Podcast claims that it was because Germanicus was too green and Tiberius's overwhelming experience and personal competence had him win out. But Augustus had ever intention for Germanicus to become emperor, his will stipulated that Tiberius adopt Germanicus and name him his heir, which happened. Unfortunately we all know what Tiberius ended up doing to Germanicus and his sons.