Short answer: he was too young and Tiberius was already entrenched in his role as heir.

Longer answer: Tiberius is often seen as a failure and a tyrannical, mad emperor these days but one must remember perspective and the importance of readings sources against the grain. Tiberius might have been a terrible emperor but during the reign of Augustus he was an incredibly able general and a strong leader in the field. After the death of Drusus he was pretty clearly the best general Rome had and the obvious candidate for princeps. If Augustus had decided to name Germanicus heir and put aside Tiberius he would have been leaving a young and relatively inexperienced politically emperor with an older, better connected and likely angry rival still in Rome with no real reason for the decision. After fighting through all the civil wars it is unlikely Augustus wouldn’t have seen this as a problem.

The idea that Augustus did not like Tiberius is hard to pin down with certainty. First of all, Augustus was a man who clearly did what he thought was right for Rome and most points and it is hard to imagine him putting aside Tiberius for such superficial reasons. Secondly our sources, most significantly Suetonius, have a vested interest in making Augustus (the paragon of Roman leadership) look disdainful for Tiberius (a unstable weak emperor). Thirdly, they, again especially Suetonius, undoubtedly had sources from the time but were working well after the fact. Fourthly, Roman sources are notorious for twisting facts and putting words in people’s mouths to suit their narratives. The idea of history as an attempt to determine facts is a decidedly modern idea and Romans often saw what we would call myths or allegorical stories and compelling narratives as fair game for their histories. If you want an example I would point to the speech of Calgacus before his clash with the Romans in Tacitus’s Agricola, where the Caledonian chief gives a speech that Tacitus has no way of knowing the details of with opinions that should an awful lot like those one might attribute to Tacitus himself.

The narrative of a young Alexander-type being denied the throne by an evil man and dying tragically young, likely at the hand of that same evil man, is a strong one. That is likely why it has survived so long. At best, however, Augustus probably saw Germanicus as a potential heir to Tiberius. Also I would be wary of trying to psycho-analyse any historical figure, especially one so far in the past and with so much bias and historical baggage as Augustus. It is a habit some historians like to fall into and while it is interesting to speculate my personal opinion is it is something many take too far.