Pulling this out... yeah, it is really weird that the writers decided to state "Humans, Elves, Dwarves and Halflings are the most common races in the game". They didn't need to do that.
But they did. They put forth the idea that this matters. Why does it matter? Why did they feel that need?
It would make sense if there are some settings they don't want to have all the races in. Or if in some settings the others are going to be portrayed as rare. If everything is kitchen sink then I have no idea.