I ask that people not get upset/start conflict on this post but-
My take on it is:
Airbending goes with Native Americans due to the seemingly peaceful culture. (Airbending is notable for being almost purely defensive, pacifist nature of the Air Nomads)
Earthbending should be self explanatory (hint hint... most people think that Africa is nothing but dirt/ground.) Seriously, a-lot of people aren't educated about how Africa actually differs from what the media puts out there.
Waterbending due to having the main waterbender in the series, Katar, having to put so much effort and training into her bending skills and wanting to perfect a-lot of techniques. Also let's not forget that Aang did achieve some things before Katara and she got angry. (They let their defense become their offense, turning their opponents' own forces against them)
Firebending.... I'm not completely sure on this one but maybe due to overpowering the opponent and full of rage with most of their attacks. Zuko and Azula seemed pretty angry/hot-headed most of the time. ("Fire Nation twisted this into firebending being fueled by rage, hatred, and anger")