Why are there many talented Asian-American singers on youtube but they are not mainstream?

I notice a lot of Asian singers on youtube that are actually pretty good and try to make a career out of singing but none of them have a signed contract or gets noticed by any producers. Why do you think that is? These Asian youtube singers aren’t just some random person but “famous” within the youtube community lol.

Erika David (Filipina)- Lotus Flower
http://www.youtube.com/watch?v=kaKqh7moOxs

UrbanGermany
http://www.youtube.com/watch?v=ip0-SSpLYDU&feature=related

Jason Vincent
http://www.youtube.com/watch?v=MIVu-egUMM0&feature=related
Gets a few million hits with his videos