The flow of diamonds

The rivers going west (Vaal & Orange) move diamonds downstream that eroded out of old vulcanic pipes, right? Cullingnan is on the other side of the water divide. Why don’t those rivers (running east) have diamonds in them? The “Star of South Africa”, currently in the queen’s crown, was picked up at Cullingnan I believe, so it wasn’t as if this pipe was not eroded yet.What do I miss?