First, they won't pay you unless you have made 100$, which is bad for small YouTubers. This was the main reason why I left the network.
I think, they choose small channels on purpose so they can keep a lot of money, if like 1000 very small YouTubers make 70$ each, the network can keep all the 70,000$. Because not everyone is going to reach the 100$ in their YouTube - career.