This allows the Seller's Agent to maintain their duty to maximizing the value of their client's home by reducing the commission the seller pays. While also ensuring the Seller's Agent doesn't bend to the Buyer's Agent's wishes for a higher commission to secure a deal. This also allows the Buyer's Agent to maintain their duty to their client by negotiating their commission up-front, regardless of the price of the property. They could negotiate a percentage with their client, but that percentage is the Buyer's responsibility, not the seller. It gives the Buyer more power in the commission paid to their Agent.
TLDR: Seller's had more power in pricing and commission payments to Agent's on both sides of the transaction, skewing home values higher to offset their (commonly) 6% commission payments. This segments the Seller's commission to just their Agent, and the Buyer's commission to just the buyer.