No promotions outside of the UFC and Strikforce have lesser competition because it's the truth. The reason Aoki hasn't made his way state side is that he will get beaten by the top guys here, so he hides in DREAM. When DREAM dies, he'll go to Sengoku.
Part of it though isn't even so much the guys in Strikeforce or the UFC are that much better, a lot of it IMO is that the guys he is fighting in Japan are good matches for him. stylistically the guys in Strikeforce and the UFC are much worse for him and because they want to maintain him as a star Dream will continue to protect him from those kind of match ups.