I know that the early christian missionaries to the Germanic barbarian tribes were Arianians. But Niceanian Christianity doesn't seem to have taken any root among them.
Why was it that to the extent that the Germanic tribes were christian, they were Arianians?
A few reasons.
Firstly, initial mission efforts among the Germanic tribes you are thinking of (Goths, Vandals, etc.) was conducted by non-Nicene faction during the 4th century (aka ‘Arians’). This was done during a period where the whole ‘Nicene’ question was hotly contested, so their conversion and allegiance to a non-Nicene version of Christianity also became an element of Christian and ethnic identity.
Secondly, and related, The Nicene debates were conducted primarily in Greek, primarily in the eastern half of the Empire. So the developments of that debate rarely impacted the Germanic tribes directly. They were not going to be swayed by the ins and outs of Greek theological arguments because they had little access and little interaction with that ‘world’.
Thirdly, the Germanic tribes, even when to some extent ‘inside’ the Empire, were also very much ‘outside’ the Empire, and so their relation to Imperial power and policy differed significantly from the Christian church within the confines of the Empire.
Attempts were made to Nicene-ise the Germanic groups, with more or less success. Chrysostom, while bishop of Constantinople, made active efforts to bring them into the Nicene fold. The Visigoths abandoned Arianism in favour of Nicene Christianity at the Third Council of Toledo in 589.
If I may add the Goth were converted by Wulfila, a gothic captive in the roman empire wich later came back to preach the Arian Christianity to the goth. He also invented a new alphabet for the gothic language to be able to translate the bible.