Because in Norse sagas she has hair of gold, forged for her by dvarves.
Also Heimdall is called the White Asgardian, being fairest and brightest of all gods, but people usually let it slide cause Idris Elba is a very cool actor.
Also Loki and Thor are not brothers, Loki gave birth to Odin's 8-legged horse Sleipnir and is the father of great wolf Fenrir (Hela's pet in Ragnarok).
Marvel' s version of Asgard is very loosely based on Norse mythology, so many, many things differ. Also, original mythos would get R rated.