Why are Barbarians a class in DnD and not say a background or sub-race?
D&D history. Gygax first proposed them as a sub-class of fighters in Dragon Magazine back in the days of AD&D, and they became "official" with the release of Unearthed Arcana. Here's a link to the
original article if you're interested (scroll down to page 10). From a quick read-through, it appears they were basically spell-less Rangers with thief-like abilities to hide and climb in natural environments, with no trace of any "rage" mechanic.
And, yes, Gygax originally intended them to come from uncivilized tribal cultures.
The whole Rage thing always seemed weird to me. It's as if the D&D hivemind somehow transitioned from Conan as the model barbarian (sly, cunning, yet strong and quick) to Cú Chulainn, or maybe modern portrayals of Old Norse Berserkers (insane fighters utterly heedless of danger).
