Because of the selfishness that is in our hearts. Purify your heart, implies James, and you'll tame the tongue. Always protects Always trusts Always hopes Always perseveres Never fails Contrast that with bitter envy and selfish ambition and you see that the kind of wisdom James is talking about is love. Love never fails. Peacemakers are people who insert peace in the midst of strife, rather than continue the dispute.
Peacemakers are often misunderstood as weak.
Jesus certainly was. But the seeds sown in peace bring a harvest of righteousness. But fighting is put to an end when we speak with the tongue of peaceful, unselfish words, that produces the righteousness that God does desire. Lord, work in us so that what we say will consistently honor you. In Jesus' name we pray.