How Does Increasing the Number of Block Confirmations Reduce the Risk of a Successful Reorg Attack?

Each additional block mined on top of a transaction increases the amount of hash power that would need to be re-mined by an attacker to successfully execute a reorg. The probability of an attacker having enough hash power to catch up to and surpass the honest chain decreases exponentially with each confirmation.

Therefore, requiring more confirmations increases the 'cost to attack' and the security of the transaction.