Because Jesus Christ, the Messiah, is the fulfillment of all that God was teaching His chosen people through their feasts and holy days. He is the Lamb of God who takes away the sins of the world.
I understand you believe that and I hope that such believe guides you to a prosperous, peaceful and good life. However, there are significant theological differences between Christianity and Judaism. Significant Christian beliefs like original sin, a messiah as God incarnate, or that God, as a man, sacrifices himself to save people's souls from hell. Speaking of "hell", why would a loving God create a hell? And if you say "lucifer" or "satan" created it, then you believe in polytheism.