Tbh I don't think the sea that they come from matter. The point is that all of them have reached the new world and are worthy of having unlocked CoC through their trial and tribulations along the way.
Its not unfair, as Vinsmoke Judge, Hawkins, X Drake and Law don't have CoC either, and they all came from North Blue, supposedly the strongest ocean of the four Blues, yet none of them have CoC.
Kidd is also from the South Blue, and Shanks is from the West Blue, and both of them have CoC.
East Blue may seem like a lot, but its just hereditary and only coming from just 2 family lines and both of them also happen to be D's, the Gol D and the Monkey D families.