American culture is the dominant one and very strongly associated with all forms of entertainment, music included. Other accents have very strong associations too. Latin, Reggae, hip-hop, opera and country all have a preference for certain accents born from their history and public image. It's partly historic norms and partly audience acceptance. That being said, these are not static. Certain trailblazers can carve a path for new accents to become desirable within a genre but when and how that happens is uncertain.
I'm sure some accents sound better sung than others too. Any language heavy on the consonant sounds isn't going to work well in music. Singing is all about the vowels so languages and accents that emphasize vowel sounds might be preferred. That also means some accents will lose much of their character when sung. But it's really dependent on your audience.