Why did St. Anselm of Canterbury believe that only someone both divine and human could reconcile humanity with God?

In his work Cur Deus homo?, St. Anselm of Canterbury proposed the satisfaction theory of redemption to explain why the incarnation was necessary for human salvation.12

Anselm argued that humanity, being finite, committed a grave offense against the infinite God through sin. Within the framework of feudal justice, an offender must provide satisfaction—or recompense—commensurate with the status of the one offended. Because humanity could not provide sufficient satisfaction to an infinite God, the only alternative was eternal death.1

Anselm concluded that reconciliation required a mediator who was both divine and human for the following reasons:

• Humanity: Because humans were the ones who committed the sin, it was necessary for a human to provide the satisfaction.12
• Divinity: Because only God possesses the infinite capacity to overcome sin through perfect sinlessness, only a divine being could render the satisfaction required to restore the moral order.12

Thus, the death of the God-human, Christ, on the cross was the only rationally intelligible way to reconcile sinful humanity with God without repudiating divine righteousness.12