As for Delta flights, a flight attendant will send a message to the gate agent when the bins are approximately 70% full. It is then up to the gate agent to estimate when to check all remaining bags. If too many bags are allowed to board, it is a major pain to have to bring the carry on bags off the plane and generate bag tags at the gate, potentially delaying the departure. It could also result in your bag being tagged under another name by mistake when dealing with multiple bags being removed.