The Abandonment of the Gladius for the Spatha - Why?

[Thiudareiks Flavius]

The archaeological evidence from Germania indicates that the Germanics adopted the spatha as their primary sword due to influence from the Romans after the Romans had already done so. Before this they favoured short gladius-style swords, just like the Romans.

The Celts favored long slashing swords though. I didn't know the Germans favored the gladius-type sword. What about the Celts and Pics still unsubdued in Britannia ? Didn't they still wield long slashing swords (during the 1st & 2nd C. AD) ?

They did. But the reason I mentioned that the Germanics is that the suggestion is that the spatha was adopted due to the influence of barbarians in the late Roman Army. The bulk of those barbarians at that stage were Germanics (and some Huns and Indo-Iranians) rather than Celts.