Because judgment without mercy will be shown to anyone who has not been merciful. Mercy triumphs over judgment.
Our actions matter. We act from that which is in our hearts. When the heart fully receives and owns the undeserved love and mercy of God to us through His son Jesus, we will truly be changed.
In Matthew 18:21-35, we learn about the ungrateful servant who had owed much but was forgiven by his master. Seemingly forgetting the mercy that had been shown him, this same servant found someone who owed him a small amount and demanded to be repaid. In these moments he showed no mercy even though he had received great mercy himself. What happened? Did he think he deserved the forgiveness he received but others didn't?
James emphasizes the same message Jesus did in His parable. If we do not show mercy when we have been forgiven, then we will lose the gift of forgiveness that is there for us.
While we all deserve judgment for our sins, we will receive mercy instead because of Christ's sacrifice on the cross. For this, we all must come in humble gratitude before God and others. Receive your gift of forgiveness and bestow mercy on chose around you.