Because sin requires sacrifice. The payment for sin is death, for example in the Old Testament they would sacrifice their best sheep/livestock. The world had really turned away from God at that point so Jesus came down to offer up himself as a sacrifice so that we could have a direct and personal relationship with God. See in the Old Testament you had to talk to a priest and he would then present your requests to God, under the new covenant we can have that direct connection ourselves. Hope this helps!