BECAUSE JEHOVAH WANTS IT THAT WAY:
Sometimes Christians say that Jehovah could've chosen any of a billion ways to forgive sins but He chose the propitiatory sacrifice because... he wanted it to be important, he wanted to prove his love, he wanted the praise of his people etc.
The answer, of course, is that if the sacrifice is not necessary than it is no longer a sacrifice, it is a big, fat plea for attention. Let's say that I see a child about to get hit by a bus and I throw myself in front of the bus pushing him out of the way - I'm a hero and I sacrificed myself. Let's say, however that the bus is moving at about 10 mph and I could easily escort the child away from the bus but choose instead to push myself under one of the wheels. I'm not a hero and my death is not a sacrifice; instead my death is looked at as a bizarre play for attention and my story ends up as comic relief in a magazine.
If Jesus' death was unnecessary than Jehovah is either a blood-thirsty sadist or a drama queen craving attention or maybe both. In any sense this hardly seems like the behavior of the creator of all time, matter or energy.