Dr Magister
Explorer
I don't remember where I saw it, but I recall seeing some rationale that since rangers were mostly forest pathfinders and scouts, they wouldn't favor two-handled weapons (hard to swing in a polearm or greatsword in a forest) or shields (again, clumsy in a narrow woodland) so they felt that a ranger would opt for a small off-hand weapon (knife, handaxe, etc) as an alternative. They also moved rangers to light armor (studded leather) to further that archetype as a scout, skirmisher, and archer rather than the heavier armor and weapons of a fighter.
It's just occurred to me, but the heir-to-the-kingdom ranger (who definitely WAS NOT Aragorn) from Sword of Shannara (which is NOT a plot-point by plot-point rip-off of Lord of the Rings) always fought with a sword and dagger (or sword and shortsword; it's a long time since I read it). Could that be an influence?